Provided that you are sensible, don't swim out too far and stay away from rocky shores when there are waves, the Gran Canaria sea is as safe as it comes.
Sharks and beasties
There are plenty of sharks living around the Canary Islands but there has never been a shark attack. Most Canarian sharks are small and harmless and the big ones don't hunt near the shore as there are no seals or penguins. This means that there's no chance you'll get eaten by mistake.
While the sea around Gran Canaria is packed with fish, none of them are dangerous and the few poisonous ones are small and shy. The only way you'll get bitten by anything in the sea in Gran Canaria is if you poke a crab.
While we do occasionally get jellyfish on the beaches, they are mostly harmless and the lifeguard towers fly a flag if they are in the water. If you get stung, don't do anything silly like pee on it; Just head to the nearest lifeguard and they'll give you soothing cream.
Most beaches have marked swimming areas were it is safe to swim in calm shallow water. If you feel like swimming in deeper water, just don't go out beyond the buoys and you are perfectly safe. If you do snorkel in open water, use a dive buoy to make sure that boats can see you.
The north coast of Gran Canaria often has big waves, but the south coast is sheltered and calm. If there are big waves, stay away from rocky areas and don't swim if the lifeguards towers fly a red flag.