Well, you can have the sea-based character wash up on land close to where the land-based character lives, and for some reason be unable to return to the sea - then the land-based character would be in a position to help the sea-based character. Maybe their shared goal could be to help the sea-based character go back to the sea, but to do so they have to seek out someone or something that lives on land?
Maybe the sea-based character is cursed and can no longer live in the sea, so they must lift the curse. Maybe something has been stolen from the sea-based civilisation and taken onto land, and the sea-character has to/wants to go after it, and the land-character decides to help them. Maybe something bad is happening to the sea-civilisation, and the answers lie on land.
There are any number of possibilities in getting them to meet, and making at least one of them have a personality that has them predisposed to be helpful, or likely to pay their debts, or something, will mean that once they've met, they'll stick together.