This thread https://forum.atlas-games.com/t/anulus-connectens-hector-of-tremere/10525/1 is a Magi of Hermes style maritime focused Tremere magus it has many ideas that might be worth stealing.
Edit:
Your restriction is pretty brutal and I'd expect it to lead to the creation of an enchanted item or two that allow access to seawater but still give the game master enough space to perhaps still make a story from it. Do I understand correctly that in order to cast the character must either be not on dry land or be touching sea water? The phrasing "Not touching sea water or on dry land" seems to indicate that they could cast neither while or dry land nor while they are touching seawater which would be more restrictive yet.