This one has gone unanswered so I'll take a stab at it. I would take the boat out and break in the motor as is. Then once breakin is done and you've done your break-in oil change, I would run the boat at Wide Open Throttle (WOT) and look at the MAX RPM's. Compare that the Max. RPM range for the engine as published by the manufacturer with what you're getting for a Max. RPM at WOT. If the RPM's at WOT are under the MAX. RPM range from the manufacturer then you are OVER propped. If your RPM at WOT is OVER the MAX. RPM range from the manufacturer then you are UNDER propped.
If you are anywhere within the recommended range, then you are typically fine. Optimally, you should be able to get to right up to but not quite equal to (and certainly not over) the Max. RPM range. However, the cost to eek 100-300 rpm's as compared to the benefit might not be cost effective. Loading the engine is bad and over revving is bad. This is easiest and most common way to get a baseline and determine how to proceed. Plus, it only costs you a boat ride. I know that a boat ride is a miserable thing to have to do, but sometimes you have to buckle down and do the undesirable