I've done the flight from Marathon Key (probably a better choice for most people than Key West) to the Caymans. It's correct that you need to file IFR and for that you should be instrument rated, but someone commented that "you would be" flying VFR. I am here to say that isn't necessarily the case.
When you're flying that far over water, you're not going to be interested in flying low. You want options, or at least TIME, if things go wrong. Time to radio your position, time to analyze the situation, time to react, time to prepare. That requires altitude. Altitude requires climb, and climb as quickly as possible if you really want the best safety you can get.
On my flight to the island, I had to descend through clouds, but was mostly VFR for the whole trip there. A little climb through the clouds coming out of Marathon to get on top and fly over Cuba. I also had to fly part of the instrument approach into Grand Cayman.
On my return flight, I had to climb for a while, actually, through many build ups. It was a bumpy ride in the clouds, too, but we set up for best climb and headed up to get above everything. The clouds sometimes hang out fairly low over the Caribbean and Bahamas too.
Do yourself a favor...get the instrument rating. It's worth it if you're going to do extensive travels by air anyway.
Other than that, the flight was not extraordinarily expensive, or complicated. As mentioned in the thread, go the Jim's page, become a member at least for a year, and you can get a Cuban overflight permit plenty cheap. The worst part about the whole thing is the psychology you go through after you pass Cuba and are out of glide range, knowing that if you go down, it's going to be a LONG time that you may be floating waiting for help, IF they can find you, and that's IF you are able to extract yourself and family out of the plane. THAT's the hard part...not letting your mind get to you. It was literally the first time I ever actually wished we had a whole-plane parachute like a Cirrus. Other than for over-water, I really could care less about the chute, but it sure would be nice for water ditchings.