If the sailboat is intended to be a major compnent of the story, definietly do some research into real world transoceanic cruising, both planned and accidental :P

Consider a catamaran. The twin hull design might give the couples more privacy (ie the couples can each have their own "hull") with the central saloon for most encounters. The shallow draft means you can do more coastal stuff as well. Search the web for "megayacht" and "catamaran" for examples. I'd think something in the 40+ foot range for a catamaran would be roomy enough with adequate storage for extended periods. A years supply of food for 4 adults is gonna be tough however. Sailing is your only option as no yacht can carry enough fuel for very long (well, any yacht that can be run by 4 people anyway).

You can have lots of fun with small personal watercraft like hobie cats, windsurfers, even jetskis if they can spare the fuel. The larger yachts can easily carry that kind of stuff.

Weaponry would depend on the world situation. Lots of ports frown SEVERELY on heavily armed personal watercraft in their ports. Also, it would be tough for just 4 people to fend of a serious pirate attack (think 2-3 motorboats loaded with folks with automatic rifles) barring a mounted gun emplacement which would DEFINITELY be a no-no in almost any port.

If you want to be REALLY sci-fi try a submarine yacht from ussubmarines.com (think that is the address). Obviously they will need a fuel depot somewhere, but how cool are those things??