The good news is, that's not blood in the water. The bad news is, it could still be a sign of danger. Red tides can be as beautiful as they are toxic. Typically caused by dinoflagellates, these colorful blooms are often associated with the production of natural toxins that can kill marine creatures, and harm people too.
The colorization comes from the algae's pigmented cells, which can turn the water red, brown, green, pink or even purple. Not all red tides are toxic — many can be perfectly safe to swim through (if you don't mind wading through murky mats of microorganisms) — but swimmers should always exercise caution before diving into any algal bloom.