dont bother with a 4.3 v6 from GM. yes, its agteat motor, all of my s10/sonoma's have had them however, my new fullsize 1/2 ton GMC with the 5.3 v8 gets better mileage then the sonoma could have ever wished with. this is also accounting for the same 3.73 rear end. I get about 19/20 empty on the highway with the truck and with a 19' bayliner behind me and the a/c on it only drops down to about 14/15 runnning 65 in it. you can find the older 1/2 chevys for a decent price now-a-days.
the other thing to remember is even though your little diesel VW pulls the boat fine, backing it up is going to be a pain in the butt since the wheelbase is so short. if you can, find a older Dodge cummins truck with 150K on for cheap and just use that. You'll be able to pull whatever you want and still get good mileage. our 06 cummins gets 30 on the highway empty.