Didn't the C only come in commercial ratings, with 1800rpm max rather than 2300 and obviously much lower power? Just going by heart, mind. I looked in my specs sheets archive, but didn't find it...
All the boat sellers advertise 1300 HP for their 3412Cs. I am sure they are going by their spec sheets fro CAT
By boat sellers, you mean brokers, I suppose. In which case, if you think they check facts before advertising a boat, well, think again. There are exceptions, mind. But they are just that: exceptions (few!), rather than the norm.
Have seen several different ratings from 650hp to 800hp. A later 3412C version did have 1350 hp. The 3412E' rated at 1400hp.
Each engine tune has 2 ratings; SAE or Gross HP at the flywheel and shaft HP. Failed used car sales people, now selling boats give the larger numbers, MPH and SAE HP. Real boat brokers usually comment in Knots and shaft HP. This usually explains most issues when comparing engines (or boat sales people).