it's all about the grades of the roads you'll be traveling.
do you know what kind of boat ramps you'll be using? nice, consistent, the same one(s)? or lots of different ones?
the FWD on boat ramp is definitely an interesting question. due to the angle you'll have less weight on the front tires as well. it's going to greatly depend on the boat ramps used. some places have really sweet, smooth, perfect, low grade boat ramps. i have no doubt you'll come right up the really nice ones. others have really wonky, tight, slight curve, very steep, and big drop offs for the boats...i wouldn't want to have FWD on the one i used earlier this year.
should be easy enough to google it though - i'm sure lots of RWD trucks have comments about pulling boats up ramps in RWD verses 4WD - i'd look for those kinds of discussions.
if you're towing flatland you can do whatever you want. flatland = weak sauce for towing. i wouldn't think twice about it. a properly balanced huge trailer can be moved ***by hand*** on a flat parking lot. pretty amazing actually. even a small engine easily moves huge loads on relatively flat ground. i've towed other cars, 16 foot car haulers (i do not recommend that at all) with Subaru's...in mostly flat areas.
as soon as you start driving in the mountains everything changes. have you ever ridden a bike - the difference between flat and a grade is enormous - same thing on your vehicle.
also - when are you likely to be towing? boats are often towed in the summer which = heat which is hardest on fluids, transmissions, engines. that's the problem for me towing a boat - i always want to use it when it's like 85+ outside which is always harder on the car. on 2000-2004 Outbacks the cooling system is the weak part, but i'm towing a huge boat through the mountains.