Swamps need to be given a wide berth when sailing. Draugr will spawn in the swamp shallows and if you're close enough they will agro and walk out to deeper water, and yes, they can attack you from under water so the point is: Avoid situations where you get close enough to agro them (generally will mean you avoid getting hung up on submerged terrain) and if you still do find yourself in such a situation again, get unstuck and paddling/sailing out of range as quickly as freaking possible, and serpentine to mess with their aim.

When it comes to swamp I'm generally sailing far enough offshore that my map doesn't render the swamp. I might jink inshore close enough to render the coastline on the map, then back out to a safe distance. I'll approximate a marker on the unexplored area if I spot a crypt from sea.