I'm not too sure about outboards but diesels are rated according to their design specification, eg. pleasure, commercial etc. This is typically given as x amount of hours at full throttle in a y time period and, also, an engine speed which can be maintained continuously. For my KAD 300, recommended max rpm -200rpm is the continuous use rating, which, I guess, is the max permissible cruising rpm.
I reckon that's still got to be stressing the engine a bit and I've settled on approx
-900rpm for a balance of performance against longevity for normal use and -500rpm for when I'm in a hurry.
And then, of course, there will be those occasional blasts flat out just cos' I like it. However, I intend not to break it, ever.
For outboards, I reckon cruising at about the max torque rpm wouldn't be a bad thing.