There are certain limitations, such as the use of "Blast" effect spells in pressurized environments. This is a GM thing, but if a player uses a spell that changes the air pressure, it can cause breaches or if those do not occur, the pressure changes inside the habitat can set off alarms or self-sealing bulkheads.
Most sea-faring magicians use Sea spirits' Guard power to prevent all kinds of sea-type nastiness. Their greater form Divination ability is even more so, with the ability to divine the general hazards or locations of nesting Kraken and Megalodons.
The interior of a ship always summons a Hearth Spirit, while above decks is Sky and Sea. Visibility on the astral plane under water is virtually nil because of all the life in the water.
One little tidbit about T: WL is that (while not directly related to sea travel) when the sun is up for 3 months in the Arctic, summoning a nature spirit will last until "sunset" which can be months away. Also, certain shamans like Sun or Moon, may be considerably more powerful (or virtually useless)
Other than that, most of the underwater rules are explained in Cannon Companion. There really are no restrictions on above water casting, save for some of the nasty toxic zones like most of the North Sea.
