Why is this?

Technically, it's standing water. If a devil 水果 user is knee deep in a pond 或者 lake, they'll drown. But we don't see that since the only body of water around in One Piece is the ocean.

I remember 阅读 the issue about water weakening a DF user somewhere, don't remember if it was in the 日本漫画 或者 wiki though. What Exorcist 说 is technically correct but there have been scenes in the 日本漫画 that confuse me. As far as the case of sea stone, i guess we'll have to wait until Vegapunk is revealed and explains it 更多 seeing that he was the person that 'discovered' its effects.