With this new power comes a weakness: the sea rejects the new Devil Fruit user and they become helpless while submerged in water (or "becomes a hammer," in Japanese parlance). Oda stated in SBS that Devil Fruit users are susceptible to all types of water, not just seawater.
He elaborated on this, saying that "moving" water, like rain or waves, does not weaken Devil Fruit users, while standing water does.
Oda also said in SBS that it isn't until a Devil Fruit user is knee deep in water that they become immobilized, this is seen with the Gorgon sisters and Luffy in Boa Hancock's bath. Brook was also seen slowly losing his strength as the water levels in the flood Ryugu Palace, demonstrating that being in contact with the sea does not instantly rob their strength unless it reaches a certain level. Even if a part of the user is submerged, they can not 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; i.e., when Luffy was knocked into the pool at Arlong Park, Genzo and Nojiko dove underwater and stretched his neck so his head would be above water. Being weak against the sea proves deadly to many Devil Fruit users who are pirates and Marines sailing the Grand Line. As well as this, the use of a Devil Fruit puts users at odds with races like the fishmen and some with greater weakness to the sea than normal (like the Kage Kage no Mi whose zombies are purified with salt) are almost useless against these races. However, the Devil Fruit power will also render a Fishman or Merman just as weak in water as anyone else, so this advantage is only applied to one that has not consumed a devil fruit themselves.