We spend a lot of time cruising for long weekends with our closest friends and family. It only takes a couple of days in a 500 sf home to really figure out what drives you nuts in someone! It works, because it has a clear end date and then we look forward to having them back.
While you are looking for cruising partners, you are still looking for a partnership of sorts. Its unclear whether both couples would own the boat in your query. Regardless, a partnership is always the first ship to sink. The most important agreement to iron out, is how to end the partnership. How does each couple get out? Can one require the other to buy them out? Can you just leave?
Whether each partner has the ability to take advantage of an exit is even more important. Just agreeing that one must buy the other out at a pre-determined price is useless if either party doesn't have the money to do so. It becomes like dealing with a cornered animal, if one wants out, but can't do it.
They can work, but be sure to have a rock solid pre-nup.