Without waterbreathing I can't keep my head over water in deep water (swimming apparently only works in shallow waters), but I did once swim from Arlinor's Retreat (sp?) to the northern tip of Thestra taking rivers the whole way.
That was one of my favorite journeys I took in this game so far. It was beautiful and peaceful (and sometimes deadly), and it's nice to see how many unobvious content they put in the game.
I look forward to someday being able to swim under the ocean with you, Dennar
(Maybe they'll make water breathing rings for warriors.)