Con and maybe Agi I would say. Strength doesn't matter as much as endurance. Not to forget that strength has enough uses right now, while con only helps with health (just like strength, though it has twice the effect of strength.)
Edit: What about the possibility of making swimming a skill? It would keep newbies from walking right into the water and killing themselves, hopefully giving them enough play time before they get it to learn that it is horribly dangerous.