All images show one or both of the following.
Freshly inundated red gum woodland within Aldinga Scrub CP, and the old wooden fence posts that remain as historical evidence that the area was once cleared for farming and timber extraction.
Good rain during the preceding days with more to come as I type.

ATTENTION please,anyone still interested in the ID for this weird polychaete worm which I first submitted as this single image on 24 Feb 2018(photos were all taken on the one shallow snorkel on 23 Sept 2010) is encouraged to revisit my original observation which now has 5 images not one image. [And I've got more pics to add but I've explained that in a comment under the initial observation]

Grass clingfish,one of the two more common sorts at this 'hotspot' shallow shore dive site in the past,seasonally and erratically depending on local catchment status and mean rainfall pa etc.Via scuba.Poor focus,not a good photo at all.But these little green fish of seagrass outliers have been harder to find in each warm season,both when I took this pic and later, following the wettest year on record in Southern Oz for many decades,which occurred in 2015-6,and extended in many coastal areas of SA into early 2017.Of course,much of Oz is now dry,with below average rainfall.Should be a better dive season locally come spring summer autumn than it was for past 2 years or so,which were the worst for bad inshore water clarity,nutrient excess and 'sudden freshwater dump' decimation of sea-grass meadows, and denuding of rock reef algal canopies ,that I can recall in my local SA diving lifetime. The local reefs beaches and meadows had not fully recovered from several almost as unseasonally wet weather phases within the previous 1-2 decades,which bodes ill for next generation of SA divers.And all local beaches have less sand than in anyone's living memory(excepting nearer metro Adelaide where active replenishment via truck and slurry pipes is occurring,at some considerable but necessary expense to taxpayers).

4 images all of the one clump of brown algae attached to rubble bottom @ 6m depth between old and new jetties.
Scuba dive.
This is a common alga in coastal marine SA.
It also grows on many of the jetty piles, mainly the lower halves so at Rapid Bay the main depth range for this alga is between 3 and 8m.
I find it both attractive and frustrating.
It is quite ornamental on close viewing.
But I can stare for minutes at a single clump like this one, expecting to see at least some sort of small, highly evolved, strongly associated and very cryptic fish species such as one of the poorly researched 'kelp clingfish', but I never do !
(Very occasionally I might see 1 or 2 juvenile pipefish in Stigmatopora genus, but even they are as or more likely to be observed on numerous other types of algae ,and on or in seagrass.)
But I shall persist ,because having learned how very difficult it is to spot 'grass clingfish' on seagrass blades (e.g. at Normanville 'hotspot' there are at least 3 species,but even in clear calm sunlit shallows getting an image is almost impossible because the instant a lens is pointed at them they duck behind the blade where they are next to invisible to the diver and are still easily able to keep an eye on you by peering around one edge of the narrow green blade!
Swimming in ever accelerating circles around a small seagrass outlier clump that you KNOW is home to several grass clingfish because they've ogled you from a safe distance as you approached, is a complete waste of effort and I imagine they must be snickering as you make an utter fool of yourself !!
Plan B is useless, too: gently using a gloved hand to encircle every green blade in one of the smallest clumps of seagrass, still with 1 or more clingfish hiding in it (and with nowhere for them to flee to,because such outlier seedling clumps are usually surrounded by a desert of white sand populated by 'nasties' e.g. crabs, flatheads, and goatfish) and slowly 'milking ' the hand upwards does force the little greenies towards the upper tips of their chosen blade(s)....but asa my hand has reached the upper ends of the tallest grass blades the clingfish ,with an amazing and sudden burst of speed, swim back down the water column and around the gloved hand ,and reinstate themselves near the base of the clump!
Full credit where credit's due: Most of these thin, tiny ,greenish brown niche habitat specialists have never even seen a diver, let alone experienced the above idiotic behaviour by this eco-diver and fish photographer, but they are VERY fast learners!
[No dry comments about diver IQ from @saschaschulz ,please !]
There was a time, when I first noticed these grass clingfish by chance while imaging pipefish in the same 'seagrass outlier clumps', that I assumed they must have another trick up their sleeve such as burying in the sand at clump's base once milked off the top of their grass blades.
But they don't, or at least I've never seen such behaviour and I now know that if I admit defeat and back off a half metre I will again find them hiding behind/among the same green blades, perhaps a bit closer to the seagrass rhizome's base, but essentially right where I first saw them!!
But...we dive 'hotspot' again tomorrow....hmm...!???!

The subject is the small green-grey-blue shiny stalked algae at low centre. I think it is a young plant of Apjohnia laetevirens but I am not sure. Whatever algae it is, it was not rare but rather uncommon, during this ~14 daylong dedicated dive and snorkel trip to this remote offshore island group. When I did encounter it, it was usually in shallowest sub-tidal depths. I think this image was via snorkel in Groper Bay, Flinders Island, but I might have been doing a shallow shore based scuba dive, because we [NB: My buddy was top-order, in the form of Scoresby Shepherd. You betcha I'm boasting, loudly and proudly. What a privilege ! ] tended to alternate between scuba and snorkeling while sharing communal rental quarters just above the beach for that part of the trip.

Subject at centre. Irregularly lumpy quite extensive soft invertebrate colony on upper section of large rock's almost vertical overhang with heavy shade just beneath this level. Shallow subtidal snorkel at cliffs base,Carrickalinga. Shore entry via walking track from North Bay car park.(This is part of a Green Sanctuary Zone in the Carrickalinga Cliffs MPA since 2014.Personal observation: reef species and commercial scalefish numbers and sizes have rallied noticeably within the 4 years since this Green Zone was gazetted.I do not have photos of the example I'm about to give but it 'says it all' as far as I'm concerned: In January 2017 I watched a dive buddy from UK 'play with' a large male Bluethroat Wrasse for some minutes.This was during a boat dive midway between Myponga and Carrickalinga beaches i.e. in middle of the 'no take' Green Zone .He was enthralled by the size, colors and inquisitive antics of the wrasse which showed no fear and kept returning to check him out! I had never seen mature male Bluethroat Wrasse behave like that anywhere in my LGA (which includes all of Yankalilla Bay and Coweelunga Bay) before that.I had only seen such behaviour in this species when I was diving and snorkeling on remote offshore island reefs rarely visited by fishers and spearos.The obvious reason being the dominant males are first to take the hook or spear so the attrition rate is much higher for males than females. Being wrasses and being strongly site associated the one breeding male for each harem often enjoys a very short reproductively active life and seldom attains full size.The largest female transitions to assume male role then suffers same fate.I detail this for 3 reasons.#1:Green Zones do achieve great things and quickly.#2:We soon realize the 'big' male wrasses we'd been thinking were at or near the species' maximum length were actually quite small.And #3:Apply this scenario to SA's largest site-associated(territorial) bony fish of rock reefs , the iconic Western Blue Groper (a wrasse)and it becomes obvious that species needs state-wide full protection status urgently.I do not exaggerate in saying the health and balance of life both plant and animal on virtually all reefs in SA depends on the viability of our top order wrasses.(Apologies for preaching to the many converted on iNat but I like to think at least a few new participants might learn from we oldies where shifting baselines are concerned😑)

No close photos but the 2nd photo incidentally includes a helicopter hovering low over the beach. SLSC event with many participants in and on the water so the presence of dolphins led to a precautionary fly-in to confirm absence of sharks.