It depends.
In a pool, no. They all took swimming lessons since they were little, and are competent swimmers.
On a boat, yes. Anyone could slip and fall off. The ocean is a very different creature, if you will. Currents, rip tides, weather events can all make the ocean a dangerous place.
At the seaside, no, if I know the area. We live coastal, every year you hear about a person or two who over-estimated their abilities and got caught in a rip tide or otherwise drowned. My kids tend not to go out very far, and stay in known safe areas.
I know someone who almost drowned in a rip tide. She's very athletic and been a competent swimmer since childhood. I never would have thought she'd get in trouble--she probably didn't, either.