I get a loud beep and the volt light comes on.

[ESPNRuss]

2003 Sportster 4Tech I get a loud beep and the volt light comes on. The boat continues to run fine. This happens at various intravals, sometimes I ride for 20 minutes, other times as early as a few minutes. I just bought a new battery and it tests fine.

 

[Eclipse1701d]

Could indicate a bad rectifier...

 

[Albysure]

like eclipse said but follow all your grounds that connect to the engine and look at the washers at those spots...if the washers are rusty you will not get a good connection...first time this happened to me it was a rusted out washer....take it off and clean the area really good and put a marine grease of your choice in the area and on the new washer and ground end....hopefully its a simple fix like that...
it sounds like it cause you said its running fine...battery is staying charged? if so then you got a spot where ground is not getting a ground to the engine...