Barotrauma a go?It has single- and multiplayer option.
You can have someone build you a submarine (Designed for single or multiplayer). Talented people can make automated systems.
There are 4 factions you can build rep with (Including the Clowns). All with their own shops and agendas.
Random (story) encounters on bases. And between.
Option for creeping radiation on map. Ala FTL. Forcing you ever deeper (towards right side of map).
Exploring and looting caves, wrecks, bases, alien ruins and enemy subs (after killing them).
Did I mention the cute little pets you can feed bodies to?
Only downside is the limited amount of loot a single character can carry, when looting. Compared to how much loot can be found.
https://store.steampowered.com/app/602960/Barotrauma/
