In Europe we tow RIB's and other boats with normal cars or small to midsize SUV's. It's a particularly North American thing to require a 5.4 Litre V8 pickup truck to tow a medium sized boat!
So for example I used to tow a 6.5m RIB with a Ford Mondeo (aka Contour in Canada). That was about the limit for that sized car. When I had a 7.5m boat and trailer (around 2250kg all up) I used an Isuzu Trooper SUV.
Your Passat should be more than man enough for that RIB imho. Only problem will be on slips that are slippery/beach launches etc. Thats when 4x4 comes into its own. Do a search, there have been numerous threads on towing and towcars on rib.net