The Seaborn Sorcerer archetype replaces the standard Aquatic bloodline arcana with a power that grants +1 caster level "When you are in a body of water large enough to float in..."
The question is, if you are on a boat, are you "in" a body of water?
Full disclosure, this is for Skulls & Shackles, so the character in question would gain the benefit quite a bit if allowed to activate it on deck.
Maybe I'll delay activation on deck until a later level...
I think it is an interesting question, as the line isn't cut and certainly not dry^^
I assume by « in », the rules don't mean covered on all sides by said body since it applies for us humans simply swimming at the surface.
Let's complicate the question a little bit.
What if your are on a ship, below deck, below the flotation line?
Do we have to be partly in contact with said body of water?
What if you are in a submersible during a dive?
What of a diving suit? A soft one? A hard one?