Dex is good, but IMO it's not as vital as Con. Sailors were dying a LOT, and it wasn't from accidents. Con would help a lot with diseases (those were rather common on sailing ships), the exhaustion inherent to working on a sailing ship and the lack of nutritious food. And scorbut, which is a disease, was one of the biggest issues in intercontinental sailing.

Darkvision might not do jack for navigation, but it would be very useful when you need to work day and night and the only available light sources are torches, oil lamps, candles, moonlight and starlight.

The OP never even mentioned pirates.

AFAIK most sailors couldn't swim at all, and when would the lower speed actually be an issue? In most cases, it's all about whether you get exhausted before you're at your destination.

Great point with the submarines, though.