my next project would be like this and i would fake AI activity for objects which too far away from player to see or interact, ai would be written with functions and without behaviour tree.
about your submarine, do you really need them to be for thousand km from your current point? can they be, let’s say 2m long and in 2 km? do they really need to be there, if they are submarines, you probably dont see them, so only on map, you can have object in memory, which represent submarine and his stats and this object would have position saved, then you can draw this position on your map.
Dont see any issues, at all, but mb i imagine your game differently(for sure).