John, if you have an M25 series engine most likely an M25XP in a 1989 C30, then that alternator is OEM and should (I say should not is) most likely aligned pretty well. Start with a good belt and tighten it properly and drive your boat around before the end of this season. When the old belt is off, get a simple straight edge (a cut down yardstick will do just fine) and check the alignment by placing it on the crankshaft pulley and the alternator pulley and see if they line up. They usually do on the old OEM alternators. If they don't just use a couple of washers on the curved support bracket where it connects to the engine under the thermostat and at the alternator bottom (you won't be able to get anything else in on the upper 2 inch foot). Check to see that your support bracket is straight. Take the belt off and bring it with you to the store or else you'll be driven crazy finding the right length. 3/8 inch belts. When you've dropped the alternator, make sure the nuts on the studs holding the bracket onto the side of the engine are all snug. Also check the integrity of the weld at the upper corner of the bracket itself, at that 45 degree angle gusset at the upper portion of the bracket at the 90 degree turn.
This is a less than one hour job if you have the new belt. Buy two belts, and write down the #s. This is really not a winter job, 'cuz you need to run the engine once you're done or you won't know til next year.
With your symptoms, I would bet more on belt tension than alignment. It's an easier fix, but neither one is rocket science. And you can check out both very easily. Don't assume it's just one of the culprits.
And put the belt on the correct way, ridges IN.
