Normally, if the player swims at "normal" pace then his/her fatigue will slowly increase over time. However, if the player presses the "walk" key (default W + S) then the player is able to still swim at normal pace but without gaining any fatigue at all (similar to how walking on land doesn't gain fatigue).
- Legacy ID
Press "walk" key (default W + S) while in water and then swim.
I discovered this issue while using "Dslyecxi's Stamina Bar" mod.
Humorous, as there's few available sea worthy methods for deploying as of yet. But you are likely correct, in that there's no fatigue. On the flip, why would you want to deter further use of sea assets at this point of the game? ;-)
Yeah, as an issue it definitely ranks low on importance compared to other issues. However, I figured it's still worth identifying since it's clearly not an intended "feature" by the developers.