User blog comment:Dark Doctor/Jinbe vs. Luffy/@comment-3242083-20110616213254/@comment-3242083-20110616225833

Indeed...he couldn't breathe underwater.

Luffy can't breath underwater + loses complete control over his body if he is in knee-deep water. I'm sure if Jinbe wants to he could kill him at any point. Probably not on land, but we speak about 10.000 meters below the sea surface here, the only thing between them is a lousy bubble.