Harry casts spells in at least one sequence while on a boat. I don't recall him casting any while actually in the water though. Casting a spell that has an effect on the water around him doesn't seem to tax him any more than any similar spell on land. One key thing to note would probably be where the spell effect is targeted. It's fine if it has an area of effect but I'm guessing spellcasting from one ship to another would be problematic. This is all guesswork however.
There seems to be some inconsistency with respect to running water. Harry does not seem to have a whole lot of trouble with his spells while on board a boat. But proximity to running water in YS seems to have some sort of barrier to magic, enough for a Threshold while on a bridge across a river it appears.
I have a scenario coming up, where my group will be investigating a russian freighter that is dead in the water. The 'weapon' it's carrying is actually the bones of Baba Yaga, coming to America to spread her evil and sorcerous ways. My question is this: On board a ship, in the middle of the ocean, is there any penalty for spellcasting? Or anything regarding thresholds over water? I know *running water* can cause spellcasting issues, but what about the ocean?
I do like the idea of the Russian Freighter bearing the bones of Baba Yaga, but I have to ask - if they are being brought over as a magical weapon of sorts, why wouldn't those involved transport them through the Never Never? I'm not saying the ship's an invalid option by any means, I'm just curious.Some great ideas and images there, babel2uk. But in answer to your question, the bones were found by a group of cultists. Cultists with just enough magical knowledge to recognize what they have, but not enough to fully understand it (or use the NeverNever, for that matter). 'Borrowing' the flesh of the rats and seamen is exactly what she plans on doing (has done, by the time the PC's catch up with her).
The ship idea has great possibilities for a straight up horror scenario too. Taking the idea from FM's post above about drawing power from the people on board the ship rather than the magical energies of the environment, I quite like the idea of the bones starting to reanimate, rebuilding the Crone using flesh and blood from the people on board. If each time she does it she's drawing power from the rest of the living on board you could run symptoms a little like radiation sickness spreading among the crew - just to muddy the waters as to whether the occult threat is the only dangerous thing aboard... first the rats start dying off, then the ships cat. After that one of the crew in the ship's sick bay is found shriveled and eviscerated, then everyone starts experiencing headaches and nausea, and the body of the ship's engineer is found. Yes, that's right, the engineer who was trying desperately to get the engines started again. Now would be a great time to call the coastguard, but around the time everyone got sick all the electrics on board ship went out. So now you've got to hunt through the bowels of the ship with some flashlights and cigarette lighters, and the flashlights are starting to fail....
Actually, maybe I just answered my own question as to why they were sent over water rather than through the Never Never. Maybe they're one of those weapons that you just don't want to be holding when it goes off.
the bones were found by a group of cultists. Cultists with just enough magical knowledge to recognize what they have, but not enough to fully understand it (or use the NeverNever, for that matter).