Why is this?

Technically, it's standing water. If a devil フルーツ user is knee deep in a pond または 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 読書 the issue about water weakening a DF user somewhere, don't remember if it was in the マンガ または wiki though. What Exorcist 発言しました is technically correct but there have been scenes in the マンガ 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 もっと見る seeing that he was the person that 'discovered' its effects.