User blog comment:Yonkou Rome/Devil Fruit Logia - Water?/@comment-1595565-20101016211145/@comment-69.251.32.183-20130407051302

Natural (pre-existing) water should affect the user as normal (standard DF debelitation), but their own water shouldn't harm them.

Also, just managing to trap other DF users in water doesn't guarantee a win except in a one-on-one fight, as the opponent's allies might help them escape the "Water Prison".

Also, the weaknesses should balance out the strengths- namely a weakness against both Eneru and Aokiji's DF's, and a stalemate against Crocodile's (drying up moisture vs near-infinite water supply means the powers pretty much cancel out, leaving victory in the hands of the stronger physical combatant). The only DF it could possobly win against in a straight fight would likely be the Mera Mera no Mi/Flame Flame Fruit.

All in all, a Water Logia user would have to be rather tricky, since s/he would only have access to a few purely offensive attacks (water bullets, pressurized jets, etc), instead being forced to win most fights with mobility alone (by managing to engulf the opponent).