2.5inch spinning disks max out at 5Gb, so for anything bigger you’d be looking at spendy SSDs or 3.5inch drives.
3,5inch won’t be straightforward, but not impossible, you could use a USB to SATA lead, and feed the drive the 12v it needs from whatever supply you have on the boat.
So many options here;
Just press CTRL+D
or use gmail and tags, then archive the messages so they don’t ‘take up space’
or a notepad text doc in dropbox
or google keep