Rob , If you install a 5-flat on your truck (with the blue wire tied to the B.U. lites) you would be set to tow your buddy's boat .
You could either put a matching 5-flat male on your boat trailer using just the four wires you need or it would be simple to make up (or purchase)a 5-flat to 4-flat adapter for your boat (just omitt the blue wire)
for towing your rig . I guess it comes down to which ever is simplest/cheapest?

Ken