From what I can remember from the first game, there were deep underwater chests you could lockpick to gain forgotten treasures.
You could dive to look for secret passageways that lead to unknown parts of the world, and apparently even to this day there are still places that players have yet to discover in MO1.
If you don't want to fish, you could go head to head against the marine life that will likely have some that are aggressive.
Obviously, we don't have these yet in MO2, but we may see them at some point in future.
For now, swimming underwater and coming up for air when needed is a viable way to avoid being seen by those on ground level when travelling. Those on higher ground will likely still be able to see your silhouette.