Is this boat thing new, or has he been talking of it for years? Are we talking motor boat or sailing boat? Sea-going or canals?
I found getting used to a boat easier than I had expected, but only because it is a motor boat and we keep to canals and rivers in Europe.
You don't mention children, so I assume there aren't any, or others looking forward to an inheritance from you that will be diminished if the boat is only in his name.
I get the impression that you do not want to alter the way you live now, and honestly, if you are going to give living together in one house all the time a try, do please, keep both properties, and rent one out for three months. That should be more than long enough to find out whether living together is enjoyable or the reverse.
You could also suggest that he sells his house and buys the boat he dreams of and lives on it, and you keep your house.
DH and I have long since discussed that if I predecease him he will probably sell our house and move on board our motor boat. If he goes first, I shall probably sell both house and boat and find a smaller home to live in.
Do please take into consideration how you will be placed as a widow - I'm assuming your OH is the elder and we all know most men do not live as long as their wives, or if retirement goes so wrong that you opt for divorce or a separation.