Basically it all comes down to this.

FWD is easier to lose traction as it is. Why you would even consider pulling an 18 footer with your FWD VW Hatchback is beyond me. You'd have a fun time getting up the ramp with that much weight dragging you down in the rear, especially on a wet concrete surface.

Your best bet is either to get a cheap SUV/truck or just don't get a boat at all.