A boat is a really severe test. Sometimes your hull, required speed, and prop pitch make it impossible to choose your RPM to avoid a know-to-be-bad spot.
High RPM is no worse than in anything else - if it passes quickly through any lower resonances, or they're well damped.
What's fatal is running hard at a continuous RPM, which is at or very close to an "order" (multiple of frequency) without enough damping.
7,000 may be just fine for 1/2 hour, and 6,600 break the crank after 1 minute at WOT.
Sometimes this happens by using an SFI quality damper - but designed for the wrong engine. It will react (convert torsional twist into heat), but at a speed harmless to your crank, and remain passive when yours needs protection.
