For backup, maybe a blu-ray drive? I think you would want something that can withstand the salty environment, and maybe resist water. Thing is, even with BDXL discs, you only get a capacity of 100GiB each, so that’s a lot of disks.

What about an offsite backup? Your media library could live ashore (in a server at a friend’s house). You issue commands from your boat to download media, and then sync those files to your boat when it’s done. If you really need to recover from the backup, have your friend clone a disk and mail it to you.

Do you even need a backup? Would data redundancy be enough? Sure if your boat catches fire and sinks, your movies are gone, but that’s probably the least of your problems. If you just want to make sure that the salt and water doesn’t destroy your data, how about:

1. A multi-disk filesystem which can tolerate at least 1 failure
2. Regular utilities scanning for failure. BTRFS scrubs, for example.
3. Backup fresh disks kept in a salt and water resistant container (original sealed packaging), to swap any failing disk, and replicate data from any good drives remaining.
4. Documentation/practice to perform the aforementioned disk replacement, so you’re not googling manpages at sea.

This would probably be cheapest and have the least complexity.