Well, there is at least one good example of someone directly on contact with water who were still affected by someong power : the Princess of Fishmen.
If you remember, Shirahoshi-ojou-sama was bullied by the evil Vander Decken IX whose power was allowing him to never miss his target (strongly summarized). His power was still effective as he threw a insane amount of projectiles on Shirahoshi's submarine dungeon. His power enchants boththe weapons thrown and the target, so since the power is not disabled underwater, at least this power is not affected by seastone or sea presence.
We also know that demon power users in contact of sea or seastone can't use their power and that is also the reason prison's cells are made of seastone.
We can assume that :
- there is no such thing as a DF user able to use his power when he touches sea water or seastone.
- Powers are not affected if the user is not in contact of sea/seastone
- If a DF user is hit by sea/seastone, his victims (hobi-hobi no mi toy's for instace will be freed (for good or until the DF user is not in contact anymore ?).
I think this is a minor plot hole of One Piece because it is never really explained how differents types of fruits's powers work in this kind of situation. Guess we have to decide from each power.
Hope that helps.
Source : One Piece Manga and Anime — Vol. 19 Chapter 169 (p. 11) and
Episode 106, Smoker mentions that all Marine Headquarters' prisons are
made of seastone.