If you're not running at redline RPMs and near maximum loading on the engine, the information is of little utility to most people.

Someone running an over the road truck that might have a 700 HP turbocharged diesel tractor, an 18 gear transmission, and pulling 55,000 pounds of trailer up long grades and across country, might get some benefit out it.

As Jeremy started to point-out. If you have an analogous situation with at variable pitch, controllable prop AND a heavy boat AND a BIG powerplant, then maybe you'd find the EGT (exhaust gas temperature) something that you could optimize. That would be dependant on a pretty constant range of loading (seastate, wind, and PUSHING toward redline conditions).

In practice, are you really going to be adjusting the loading on the engine and the RPM to optimize things at 90-95% of capacity of the engine? Usually not, I would think.

Unless you're underpowered, you'll be at hull speed in flat water at (say) 75%- 80% of redline, with a clean bottom and prop. As you start to face a greater wind, tougher sea states, or currents. The loading will increase as you increase RPM's -- but your prop will have to be set for a balance of performance needs. In practice, you'll be setting your prop pitch more empircally.

If you could adjust the prop pitch to optimize balance the throttle setting, pitch of the prop, and loading -- then you could use EGT as a measure (not the only measure, but one measure). But this is in a constant state condition where you're paying attention to such things. Riding up and down wave trains or in a situation where your power plant is really running at 60-70% capacity, I just don't think you really would find the information useful.

If you're concerned with "benchmarking", you need to have some common criteria. In the zone you're operating, does it really make a difference is you burn 1.65 gallons/hour or 1.63?