I always keep two spare serpentine belts on board (they are specific to each engine because of the power steering pump being only on one side) and last season, before I had everything replaced, I kept a couple of new idler pulleys and a new main tensioner on board. There's no reason to not at least keep the belts on board because they are really easy to replace unless you're in rolling seas. I boat on Lake Michigan and figure I can always limp to a harbor somewhere where I can do a belt and could even do one in the middle of the lake if it's relatively calm.
The key is to have photos of the front of both engines handy so you can see how a replacement belt goes around all the pulleys. Unlike the supercharger tensioner, the main belt tensioner is not spring loaded so you just need a wrench to loosen the bolts and a 1/2 inch ratchet handle to apply some tension before you tighten the bolts back down. There's a specific torque setting for the tensioner but that's not needed for a quick replacement. Snug on the pulley's will do and you can just compare the tension with the belt on the other engine by pressing it with your thumb.
As for replacing the actual tensioner or a pulley, just take all the bolts out of the tensioner and swap it out completely. I've never actually taken one of the pulley's off myself but they're just held on by an allen bolt through the middle, so it looks easy. As I said in my last post, the tensioner on the supercharger is a bigger deal but if you lose that at sea you'll still have a running engine I believe.
My overall advice on belts and water pump impellers is try changing them once at the comfort of your dock with no pressure and then if you ever have to do it again to get back home it will be much easier than trying it for the first time while floating around. Both jobs are easy with basic tools, especially if you've done them before. Even if you don't want to do the work yourself, having the parts on board gives you a better chance of getting the repair done away from home by being able to hire a local wrench and giving him the parts.
Cheers