You can't have a hard-and-fast(or slow) rule that covers all types of boating and fuel burn. There are just too many variables. You can waste fuel in a trawler and save fuel in a motoryacht. It is all a matter of your expectations. We went from a 8mph/2gph trawler where it took us 6-ish hours to get from New Bern, NC to Morehead City, NC, (a trip of about 40 miles) to a dual 320hp/+12gph at ~20mph motoryacht that reduced travel time to 2 hours. One boat burned 12 gallons and one 24, so yes, our old trawler did save us on fuel. Do we care? Not one bit. We needed the speed and time saved to more enjoy the destination and the $35 or so that we saved in the trawler more than made up for it based on the time spent in Morehead (or wherever). HOWEVER, it doesn't scale up for long-term cruising. We can slow down and close the gap between the two boats and will do that on longer trips, but in our case. What we did and how we spend money on fuel works for us.Your friend isn't completely wrong, nor is he completely right. One thing is certain... Fuel is not the biggest concern or the greatest expense in owning a boat. Someone that shops for a boat based on fuel savings needs to understand this ahead of time.
2000 Navigator 4200 Classic
(NOT a trawler)