[Putershark]

That is Toyota's way of saying (CYA) that the brake system on the Sienna is not strong enough to handle a full blown 3500 lbs rig. In my opinion, Toyota might as well say that the Sienna is only rated to tow 1000 lbs

The State of Florida though does not require trailer brakes till you get to the 3000 lbs mark http://www.heartlandmarinefl.com/Trailer/brake.html and personally, I would not worry too much about needing brakes on such a light BMT package.

If added though, the trailer brakes are only going to give you issues from dunking them into salt water all the time, but if it would make you sleep easier... We have several boat trailer builders in the area that could guide you through the process.

I would in your position, just save my money and use it to either replace the Van's brakes pads when needed more often, OR look into a set of aftermarket brake pads for the Sienna that has a little more beef in the pad material used. The only down side to such, is that they "could" make more noise (squeaking) in the heat of summer from using a harder pad material

To me though, that would be a much more cost effective route to take AND I would not even consider putting on a harder set of pads on, till the first set of pads wore out and see just how many miles you were able to squeeze out of the first set? As a point of reference, if not driven like a race car most people can get on average about 30,000 miles out of a set of OEM front brake pads in this area, so if you get anything near such even with some light towing (heck 2 jet-skis can weight 1000 lbs and you see them on the back of cars all the time) then I would not worry much about it...