I'm looking for a word akin to 'cismontane' that refers to oceans or seas. Does this exist?
Cis- is a particle like trans-, so you can use it to construct words with some confidence that readers will understand (or be able to work out) what you mean. Not much confidence in this case, since cis- is uncommon in normal English. That said, you could try
(I am part of a small but easily amused group who would understand cispondial as meaning "on this side of the Atlantic" :-)
Keying off of Rhodri's excellent answer, I remembered the word transatlantic:
of, relating to, or situated on the other side of the Atlantic
Typing in cisatlantic shocked me because my spellchecker claimed it was a word. So I looked it up:
on the same side of the Atlantic as the speaker
Oddly, there is transpacific but no cispacific in the same dictionary. But, in the case of talking between Europe and America, you can safely use cisatlantic.