Why are you hauling a 6K lb boat every weekend instead of leaving it docked somewhere? Would it be "cheaper" to pay a marina fee and just drive a regular car down to the boat?
What about passengers? How are they getting there? In the tow vehicle? 4-5 people can be another 1,000lbs to take into consideration in addition to the 6000lb "trailer" rig.
Touregg Diesels are hard to come by. Taken off the market last year do to lack of low sulfer diesel. Plus, they run about $60K new. Then again, 550 or so FT-LB of torque can pull a heck of a lot.
To me, "can" you make a T-Egg or Cayenne work? yes. It might be on the more marginal side of things though.
What is the "dock" like where the boat goes in. I'd be concerned about the short wheelbase of the T-EGG/Cayanne and the front wheels getting air trying to pull 6-7K lbs (wet) on a lippery boat ramp, with water lapping up on your $2k muffler
and do it every weekend? Or even every other weekend?
Get the right tool for the job.. You want to pull a boat, YOU get to drive it.. F350 Dually, Diesel if you want
Load up the trailer bed with a jet ski, wet gear, etc as well.
In another unpopular vote, SUV's and kids are hard too.. Climbing up on step rails, in the wet, to reach a middle infant seat is miserable. even more so if the rear door openings are narrow. Then, as the kids get older, worrying about them opening the door into other cars (whether your Porsche in the garage, or a ******************** box at the mall). I never wanted a minivan, but a minivan is the perfect tool for the job. Simple to maintain. easy to drive. Get some power sliders and hatch and it makes life SOO much easier. Leaving room for a sliding door is a lot easier than for a full size "conventional" door. .. Besides, "one on the way" usually means even more to come
Good luck with the decisions.. Who knows, once the kid is "out" maybe the boat will be gone.