Well you have to think about why you think Nords arent strong sailors. In alot of ways they remind me of the Saxon and Vikings of Europe. Both of those cultures were sea invaders, and go destroy, plunder and leave. And when they did they took a entire army with them. So youd have to think they were good sailors. And Nords remind me of those 2.
Real life does not always equal lore, I'm afraid. While the Nords might have been based on the actual Nordic peoples back in the Daggerfall and Arena days, now they have broken off enough that we really can't assume they're vikings unless we have a source. Oh, and almost all viking raids were by a small band of soldiers, usually freelancing, and on small villages and churches. The large scale invasions didn't come until they decided to conquer England.
I'd assume the Altmer, based on lore books and their natural island status, would be the best. Not to mention their superior technology and magic. The Yokudans might have been good too, but I think the Redguards lost a bit of that over time. They make good sailors, true, but their navy isn't the best. On a person by person basis the Redguards might be the best, but the Altmer ships were superior, and they had goblin slaves to do the rope pulling and whatnot.