If I knew I was going to spend a length of time in the water I wouldn't go on the trip. Period.I agree with the surveyor. Inflatables are covenant, and cooler to wear. But If I knew I was going to in the water for any length of time a class 1 vest is what I would pick.
The conventional wisdom is that if you fall overboard offshore you're dead. If you're not dead, I seriously doubt it will be because you had a cat I jacket instead of a good inflatable or another type.
I would invest in grab handles, jacklines, harnesses (yes you are right you are better off wearing a harness/jacket than a cat I and no harness) lifelines etc.
Then I would spend your money and precious space on things like a liferaft, (portlandpudgy.com) supplies for the raft, EPIRB, survival suits if you're in cold water etc. Don't waste your space and money on a cat I jacket as you'll likely never wear it and a jacket you wear (such as a comfortable inflatable with harness) is better than the best jacket down below in the locker.