Great idea. I love that your planning the trip so far in advance that he will get some input. That's half the fun, I think. When my MIL surprised my FIL with a trip to Hawaii, she had done all the planning and just gave him the tickets for their anniversary. I would not want to be left out of the enjoyment of the reseach and deciding what to do and where to go.
As for cold water diving, my son dives in Puget Sound all the time. Once you get your wetsuit on, he says it's fine. In fact, he's out there right now. I don't know where you are in Wshington, butI know there are several scuba schools around Seattle.
Have a great time, and congratulations. We'll have our 45th in a couple of weeks, but I haven't come up with a good idea yet. Hope you have a wonderful time.