The realities, as I see them:
1. You live, work and commute in the city. This is where the bulk of your driving takes place.
2. 400-800km round trips?? You been fishing the Amazon all those wknd and not told anybody? Round trips of 400 is being pretty generous, and those are usually easy miles, on the 400/69 Towing....
3. ....a boat, motor and trailer that weighs basically nothing. It's got a plastic 5gal tank. You can easily not even have to trailer it with fuel.
If you were looking at buying a 19' Ranger or big ProV and were going to be doing a PILE OF TRAVEL, you'd be smart to buy a V8. You make maybe one or two monthly drives towing a tiny rig that weighs nothing. Towed my 17' Lund all over hell's half acre with a Jimmy, upgraded at 300,000. Tow it now with a 6 banger Explorer. My Dad and I put close to 700,000 kms on his 6 banger Explorer towing tandem axle trailer loads of rock, green wood in 8' lengths, sand, topsoil. And oh yes...a 17' Boston Whaler every wknd from Port Credit to Point au Baril. 300kms every week like clock work for over a decade. Hard, hard miles. If you think your boat stresses a regular 6 cyl pick-up or SUV to the point of burning more gas or working the tranny/motor excessively, I think you're wrong. That's just my opinion. All I can give you is perspective from a lifetime of personal experience. Not Consumers Digest, not websites, just what I've seen in real life. What you're towing, how often you tow, the distances, and where you do all your non-towing driving (the bulk of your driving, as it turns out) just don't add up to needing a v8.
Unless you're making Caesars. In which case, I'll take ten!