The sun in Australia is very strong.
Always wear a hat and sun-screen with sun protection factor 30+ and apply before and after swimming.

Always swim at patrolled beaches between the red and yellow flags. They mark the safest place to swim and the areas where lifesavers and lifeguards patrol. Areas not marked often contain dangerous rips.

If you are caught in a rip or a strong current and you are a strong swimmer, swim across it to safer waters.
Otherwise, float and raise one arm as a distress signal, until help arrives, do not panic.

Before diving into unfamiliar territory test the deepness of the water to avoid diving into a sandbar or shallow water.

Do not enter the water directly after a meal, or under the influence of drugs or alcohol.

If seized with cramp, keep the affected part perfectly still, float and raise one arm until help arrives.

Boating

Know the boating rules and local regulations, and the distress signals.

Check the weather forecast and watch the weather.

Tell someone where you are going.

Ensure engine reliability.

Guard against fire.

Do not overload the craft.

Carry adequate equipment.

Carry effective life jackets.

Carry enough fuel and water.

Do not drink alcohol.

Hazards

Watch out for:

Blue-ringed octopus Commonly found in rock pools all over Australian states. Their bite can paralyse in 15 minutes resulting in death. Unless provoked, the distinctive blue rings of this dangerous octopus may not be evident.

Box jellyfish (Marine Stingers) appear during summer (October to May) north of Great Keppel Island and Queensland.
A sting from their long tentacles can be lethal (though not always) By the time you see the jellyfish, you're likely to have been stung already. Douse stings with vinegar. Coastal beaches North of Rockhampton are prohibited at this time.

Stone Fish Poisonous stinging fish, found all around the northern coastline, beaches badly affected are normally signposted. Reduce chances of being stung by not turning over coral and rocks and wearing sand shoes.

Sharks Need I say more?

Only that If you want to avoid them;
stay out of the water at dusk and dawn,
avoid beaches where sharks are known to congregate (most patrolled beaches have shark nets),
avoid water that becomes abruptly deeper,
avoid areas of low visibility and turbid water.