I think you're okay -- you've only had the boat for a short while and can't have put many hours on that rebuilt engine yet. As a matter of fact, depending on how complete the re-build was, you're probably supposed to break it in a bit at lower revs.
But in the future rather than puttering along at 1500, go ahead and push the engine harder when you're using it. Something in the 2600-3100 range is probably about right for this engine. As far as I know, ALL the mechanics/experts agree this is better for the engine in the long run. You'll also get better charging from the alternator to your batteries.
Our philosophy is that if we have to motor, we may as well do it at flank speed -- get there sooner with a better charge to the batteries. But upon arrival, we do throttle back and cool the engine down for a few minutes before killing it completely. Usually the time it takes to drop anchor or secure the mooring pennant is about right.