I'm glad to hear that you took the course; also that you have the XC (btw, there's a separate XC forum -- this is the V70 -- which is why I wasn't sure that you had the XC even though you did mention it), which will be much better on the ramps...

re: "...Total weight is 3200-3400 Lbs I am told...."
That's at the upper limit that you car is rated for, so be careful about your load. From the practical standpoint (legality aside), if your local terrain is in the mountains (I don't know whether you're boating in mountain lakes or flatland seaside shores), you may want to invest in an auxiliary transmission cooler -- I've installed these coolers (Volvo's own) in my 240's, and together with synthetic transmission fluid it gives me a lot of peace-of-mind. On the flatlands, however, you shouldn't have a problem.

Also, if you're boating at the shore (saltwater), you may want to invest in a freshwater flushing system for your brakes (especially if they're drum-type, although disk-type would benefit, too). It's a hose distribution system -- you hook up the feed to any freshwater hose, and fresh water is squirted through your whole brake system at each wheel simultaneously.

I've got the same 4.3 V6, I/O, on my powerboat -- it's nice and frugal (I get 3.3 nm/gal at minimum speed on-plane, about 20 m/h, or ~17 knots if you're already familiar with these units)...
[funny, small boats have speedometers that show m/h or mph, but not knots which is the preferred unit of speed for vessels.]
.... Since you're buying it used (1990), though, and assuming it's a private sale (not involving a dealer), be sure to have a Mercury dealer do a check of the powertrain to make sure that your outdrive is good -- I'm not criticizing the sale (I like mine, and it's slightly older, an '89), but I don't want you to break down on the water -- things like your water pump's impeller, and that the grease in the lower unit isn't water contaminated, etc., that you couldn't check in a routine pre-purchase inspection, and that even the seller may not realize (these problems can pop up anytime -- no blame). These are just things that should be checked yearly (maybe the previous owner already had these checked at the end of last summer, and then it should already be 'good to go' this season). Or, maybe you're buying from a dealer, and then these things would be have checked, anyway.

Anyway, get back to me anytime.
Best of luck -- enjoy the summer!

Ken