I can actually answer towing the same SeaRay 205 Sport with both a 2006 V6 T4R and my 2006 V8 T4R. The boat is 21 feet long and weighs about 4,800 including toys and gas. I had four people in each 4R at the time of the test.
Conditions were mostly flat, no wind so it was a good compare. From what my butt and ears could here and feel, the V6 did just fine but felt like it was really working hard. Once up and running it would maintain speed without issue but from a stop, you better not be in a hurry. Also, no tranny cooler. The V6 for me, is great and if it was the only option I would be content.
My V8 made towing the boat mear childs play. I could take off with authority, I could pass on the freeway, and maintaining speed took little to no effort. The engine did not sound like it was straining, but working just as it should. I would imagine for a long tow, the V6 would take a beating in comparison.
In a month or so I will be driving up to San Francisco to pick up my Jeep and tow it back to Vegas. When I do this I will report back. I will be trailering the Jeep as it's a church. 1969 CJ5, 8,000 original miles on the drivetrain and 40K on the body. My father and I swapped engines and transmissions in 1984 as we found a brand new drive train. It's mint and not built by Chrysler or AMC!!! Anyway, I'll let you all know how that goes when the time comes.