I spent several years surfing around San Francisco area with a 3mm/4mm wet suit in 50-54 degree water and similar air temps (but humid and windy). Early hypothermia set in after about an hour. No reason to get out of the water, though. After 4 hours, hands were purple and hard to control; lips felt stiff, making it hard to form words. We were at the limit of our wetsuits. With 5mm wetsuits with hoods, you can go a little colder or longer.

We were IN the water; sailing you should not spend much time IN the water, so the exposure is a little different. With a spray top over your wet suit you should be able to tolerate water temps to the high 40s if air temps are similar for a couple hours. Notice, I said tolerate, not enjoy.

I've never had a dry suit, so can't comment on their limits. But I know guys have surfed in Alaska with them.