I think you've got a good boat right there! The 455 is more than adequate for almost any application, including hot rodding, really, just has to be seriously prepped for it if that is your intention. (If that is your intention, easier and cheaper to go Chebbie or Ferd anyway)
Almost all BOP (Buick/Olds/Pontiac) "Big Block" engines suffered from the same malady. Oil flow and continuous high speed operation at or near the 5000 rpm mark. Kings of torque for their size/weight, just not sustained high RPM screamers, which is what we demand from hot rodded jet boats. Short bursts at those levels are fine.
The 455 has been used in boats almost since the engine was designed, so don't let this scare you. I would get an RPM limiting CDI box and have a blast! This is your first boat, so run the hell out of it!
Ray
Edit: I prolly should clarify the Oiling problems associated with these BOP engines. From back in my hot rodding days, the problems were, as already pointed out, loss of oil in the pan (Actually, somewhat that, but more the "frothing" of what oil was there, (Foaming), and it also loaded up elsewhere). restrictor kits, etc alleviate the problem somewhat, but at the risk of starving any hydraulic lifter cam. Roller or not. Potentially other parts of the engines as well.
Back in the day on Pontiacs, we seriously deburred and smoothed all internal surfaces (Particularly on top side of the heads and lifter valley). In some cases when done, you could see a clear reflection after painting the lifter valley! A windage tray and crank scraper were mandatory, I don't recall any serious issue and we never used any restrictor kits (Don't remember if they were even available then). Our solution was to aid drainback as much as possible, and it seemed to work. Though never did a jet boat engine, but seems relevant to the discussion in progress.
