In an answer to another post, I had already provided the following information, but I thought that I should put this all in a new post so that it can be found (using an archive search) by prospective buyers.
It's rather surprisingly disappointing to have discovered this information -- the XC90 isn't as capable a tow vehicle as I had hoped. Taking the family boat or camper (modest sized) with even a small family and reasonable amount of gear may not be possible with the XC90:
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I went to Volvo's website and got their online Owners Manual for the 2006 XC90, from which the information below was extracted.
If you want to know more, or see the manual for yourself, go to
http://apps.volvocars.us/ownersdocs/2006/2006_XC90/06xc90_06b.htm#pg104
It's very informative. But here's a summary:

Maximum trailer weight without brakes, 1,700 lbs (750 Kg)

Maximum trailer weight with trailer brakes: it depends on the number of occupants (or their gear) stowed in the car! Here goes:

AWD version:
1 to 2 occupants: 5,000 lbs.
3 occupants: 4,500 lbs.
4 occupants: 3,700 lbs.
5 occupants: 2,900 lbs.
6* occupants: 1,800 lbs.
7* occupants: towing is not recommended.
( * using the optional third row of seats)

FWD version:
1 to 4 occupants: 4,000 lbs.
5 occupants: 3,700 lbs.
6 occupants: 2,500 lbs.
7 occupants: 1,400 lbs.

Other towing guidelines are also provided in that website that I cited above.
Hope this helps!