A strangely high number of the hits on this blog are searching for info on snorkeling at Port Noarlunga. Consequently for this post I will describe the site a little.
It is one of the most popular SCUBA/snorkeling/swimming sites in South Australia.
From the aerial photo it can be seen that there is a jetty which runs out to a reef.
The jetty has two sets of steps. One at the very end, near the reef, and one about half way along. That is, about a third of the way between the mid tide mark and the reef.
The reef runs parallel to the shore about 400m out. It is a natural limestone feature in temperate waters (unlike the coral reefs generally associated with the tropics) and is more than 1500m long, but only a few metres wide. I think the natural reef has been supplemented by various debris over the years. There is a gap in the southern part of the reef which allows swimmers to pass through to the seaward side if they choose to. The reef gives shelter, from the open gulf, to swimmers (and fish!) on the shoreward side, particularly at low tide. However, care should always be taken as there can be strong drift (north or south) even inside the reef. It is best to swim into the current when beginning your swim, so that later when coming home you will be aided by the current rather than have to fight it when tired. While Port Noarlunga is a popular and relatively safe area, there are often no formal life saving or other services on hand, so assesing the suitability/safety of conditions is the responsibility of the swimmer.
At high tide the waves from the gulf pass over the reef. They carry particles and bubbles, so visibility is reduced. The seaward side of the reef carries a more luxuriant layer of sea vegetation, chiefly kelp. Between the reef and the low tide mark is a sandy floor with extensive areas of weeds and sometimes shell fish.
The whole area is a marine reserve which means that bait digging, spearfishing, line fishing within 25 metres of the reef or within 50 metres of the end of the jetty, collecting or removing anything (fish, shells etc) are all prohibited activities. There is a "marine trail" with 12 underwater markers (and one anchor!) which can be followed.
What you will see.
The usual way to enter the water is to walk to the extreme end of the jetty and use the steps there. Except for the last 50m of the jetty, line fishing is allowed and you will see what fish are biting at the time. Once in the water at the steps you will see thick schools of fish (Zebra Fish, Sweep, Leather Jackets, Whiting etc). They wait there to be fed by divers and others. The Jetty legs have metal bars attached for convenient handholds and the water is about 6-9m deep here. Swimming directly out from the jetty end puts you next to the reef within a few metres. Then travelling slowly north or south will give you the opportunity to see the various sea life along the reef. Most obvious will be Old Wives, Magpie Perch (feeding on the reef) Leather Jackets, Wrasse, Bryzoans, Starfish, SeaUrchins and Sponges. Reputedly there are over 200 species of marine life on the reef, including 38 species of fish.
Of course, the reef environment is fragile, with so many people visiting it, care should be taken to avoid touching anything if possible.
If you decide to swim back to the shore from the reef (or start by approaching the reef from the shore) either keep wholly underneath the jetty or swim many metres away from it in order to give the maximum clearance to the lines of people legitimately fishing from the jetty.