There are several places which could check out your alternator. Among them:
Peter Kennedy, PKYS, Eastport (Annapolis)
Aric Euler, Rockhold Creek Marina, Deale (next to Shipwright Marina and Herrington Harbour North)
Scott Berg, Chardonnay Boatworks, Wash. DC (works Annapolis, too)
Before you call in the cavalry, check your connections carefully...especially the 12V sense wire and the field wire. Check these at the regulator and at the alternator to be sure they're clean and tight.
You could also put a multimeter on the output lug of the alternator while the engine is running to see what voltage you get. Should be about 14VDC.
Which regulator do you have?