Basically there are countless marine creatures capable of causing harm that you may encounter. However, in saying that it's also a matter of common sense. Most times people are injured by marine creatures, it's because they've done something to aggravate the animal and thus it has retaliated in defence. So, naturally common sense says that if you don't know what it is, don't touch it. If you must touch it, use your wits and apply some caution in the form of protective glove etc.
Basically, estuary environments harbour most of the little nasties you'll need to know about. Stonefish are an ugly little fish that resemble a rock - perfectly camoflaged. They have 13 (I think) vertical spines that inflict a nasty sting capable of doing you serious harm. When walking/wading in estuary areas, WEAR SHOES! Also encountered but not restricted to the estuaries are Stingrays. They're a ray (I'm sure you know what a ray is) with a long slender tail containing a single barb. Again, wear shoes when walking/wading. Stepping on a stingray isn't going to kill you, but it will cause excruciating pain and swelling on the site of the wound.
Blue-Ringed Occies are found, as already mentioned in rock pools, that are wonderful places to explore on beach headlands, rocky outcrops etc. However they are also commonly found in estuaries hiding under ledges, logs, rocks, weed or any other form of suitable structure for that matter. Don't go sticking your hands in crevices, under rocks, ledges etc. If you are bitten by one of these, you're pretty much screwed. Don't take the risk.
Asides from this lot, there are a few species of fish that have barbs of spines to watch out for if you catch them. Flathead are a long, slender, lizard looking fish. They have spines on their head, gill rakers, back and belly, which in effect, makes them a heavily armoured little beast. They certainly won't kill you, but it will give you a poke you'll remember for a while. They're also one of the most common estuary species caught and targeted.
Happy moments are a fish resembling a trevally/bream looking thing with a mottled brown appearance. They have spikes over their back, as do many fish, but the spikes are venemous. Not deadly, but they will give you some swelling and discomfort.
There's a bit of basic information on some of the more common critters anyway. If I've made errors in anything, please correct me people.
The thing to remember though, is there is no need whatsoever to worry about them. As long as you exercise caution, keep your wits about you and take the relevant protective measures necessary when out amongst it all, you shouldn't have much to worry about. Hope you enjoy your time here.
Ben