One-direction same size encryption algorithm that allows for partial replay
It sounds like you're describing a Commitment scheme.

A simple implementation would be that Alice makes a list of boat position + ":" + random 16 bytes (with a different random value per item). She sends Bob the hash of each list item at the start of the game, and later she can send Bob the plaintext of any of the hashes to prove the position of a boat is where it was when the game started. (The random value in the hash is to prevent Bob from brute-forcing the hash. If there only a small number of possible boat positions, then Bob could brute-force to reverse any hash easily if it weren't for the random value.)