In order to best enjoy the water, use common sense and remember that even small summer thunder storms can cause local dangerous conditions. Seek shelter when storm clouds become visible, and avoid the water if you see lightning. Be safe and exhibit caution if you can't swim or are a beginner.
America's only living reef absorbs the impact of large ocean swells and, under normal circumstances, Key West waters are very calm. Waves and dangerous surf conditions are extremely rare. If dangerous surf conditions exist, there is a dangerous storm nearby and you should NOT be in the water.
Your hotel or guest house will have info or materials on other water safety issues.