Well you have to realize they are tractors and as such don't have a lot of power for highway driving. You'd have to spend a lot to have one with enough power to tow a boat very far at highway speeds. A 406 or 416 might be able to be turned up to 120hp at the most. Then you're into the sbu and spending in the 40k+ range. It's not very common finding any of them with high speed axles. A u1700, 2100 or 2400 is pretty rare over here, but you're getting 170, 210 and 240hp and could tow something.
The u500s are the most viable towing/highway unimog but you're in the 100k range again. They'll run 70 topped out and tow something as well. I towed a skeeter fx21 a couple of times.