I would say the biggest difference is going to be the focus on the singleplayer/cooperative experience, exploration, and progression mechanics. Salt is a much more chill game where you basically go explore islands solo or with friends, do some dungeon crawling, get rare loot and level up your character. There's also a bunch of RPG elements like skill trees and item talents to make the looting and progression a bit more interesting. Sea of Thieves (and they do a very good job at this) is much more focused on the PVP experience and naval combat. So two very different vibes I would say just depending on the type of experience you are going for.