My 2005 Double Cab 4X4 does the job but when the ramp is slippery it needs to be in 4wd. I'm pulling an Axis A20 Wake board boat easily weighs about 5,000 lbs with trailer and full of gas.
Don't know how a van would do and don't see to many at the boat ramp?? But you'd probably be better off with a V-8 truck if you plan on pulling much, they are just much better suited for the job all around, transmission, engine, gearing, power, ect., ect..
My 02 2wd tundra pulls our 5000 pound nautique out of ramps easily as long as they are grooved. Ungrooved ramps were a challenge until i put a set of all terrains on. Also we used to have a mini van and lets put it this way front wheel open diff + tongue weight = embarassing you and your friends
saw a van struggle with a jet ski, it is easier to push the weight up the hill then pull it. My 03 AC 2wd does great and if the tires do spin I stop and click the E brake two clicks then I have no problems
How big a boat? I assume it is light due to the limit of the sienna. The AWD Sienna is nice except for the run flats will cost you a lot. They wear out quickly.
Either one should be fine on the ramps. As mentioned, tires play a big role.