Cruising speed for me is the most effecient..1.5 mpg
Looks just like this...

3500 RPM / 47 MPH around 1.15 MPG

Well on land anything below the posted speed limit is cruising :D

I cruse 3500 @ 58 MPH an I have found that to be the most fuel efficient RPM.

Yes, I am looking for serious answers. How do you determine the cruising speed of a particular boat?

Crusie speed is around 2/3 throttle. On an outboard seems to be about 4000rpm -5000rpm

Just before the secondaries open...
 

I marked my throttle lever where the secondaries just start to open (mechanical secondaries). This usually translates to about 3000 - 3200 RPM @ about 35-38 MPH indicated by the speedo.

Mopower - glad to see I'm not the only one who uses my cupholders for tool storage! :D

I did the same thing Budman did and did that ever make a difference in fuel consumption for a weekend. If we are out messing around or just going from point A to point B where I don't care how long it takes to get there. Then I have alot more for when it's play time. :D

don't know never cruised

I think it depends on how fast the boat will go. I usually try to cruise as close to top speed as possible. Unless you are in a channel and surrounded by people on the banks. Cruising speed then becomes as close to idle as possible and every once in a while it is taken out of gear and revved up just to get some more attention. I have tried to find the most fuel inefficient way to cruise, but it seems to work. The noise impresses most females and the speed makes the guys want to be you.