Personally I think this is a bad idea, even if the offline/passive mode is seperated from the normal game (and has it's own gold).
The problem with removing player interaction (the most important part) is that it really shows how empty the game is.
Whenever I go to an island to dig up a chest or whatever, I have to lookout for other ships and be fast with what I'm doing. If I see other pirates, I can decide if I want to be friendly or hostile. This is the main point of the game.
It's like Rust or DayZ.
What the "PvE" part of the game really does, is give you different ways to acquire items to sell, so you can buy nicer clothes. There is no depth to it.
The loot is actually just a tool for player interactions. That's the incentive to either play nice with others, so you can team up and do stuff together, or to fight you and keep everything for themselves.
If you remove other players and the risks that come with them, you pretty much have a ship simulator game with fetch quests, Pinata monsters, empty islands and gold that is only used for skins.
Is it fun? Sure. everyone likes having empty game seasons where they can travel freely and get loot in peace. Is it fun all the time? I doubt it. The voyages are very limited and revolve around killing skeletons and getting more gold, nothing else.
If the game was different, had actual progression, crafting and enough PvE content that doesn't just revolve around loot, then sure. I would gladly play on a PvE/private server.