My understanding was that the sea is supposed to weaken devil fruit users and rob them of their powers. The One Piece Wikia elaborated:
Devil Fruit users are susceptible to all types of water, not just seawater. This includes the white sea surrounding Skypiea. [Oda] elaborated on this, saying that "moving" water, like rain or waves, does not weaken Devil Fruit users, while standing water does.
The Wikia also states the users cannot use their powers while submerged.
Even if a part of the user is submerged, they cannot use their Devil Fruit powers, no matter how much or how little is submerged. However, if the user's body has been permanently altered by the fruit, then the user's ability can be manipulated by outside sources (eg Luffy's neck was stretched while submerged at Arlong Park).
How did Ajoki freeze the ocean?
May I emphasize he submerged his hand in the sea to use Ice Age?
How did Luffy fight Crocodile while wet at Marine Ford?
He was dripping wet after having fallen into the sea. Does the water not weaken him since he was now submerged? I assumed since sea stone worked just by touching it and it has the same effect as the ocean, then by having part of the ocean "touch" him would drain his power.
Are these just plot holes or am I missing something?