Surging idle

[BRIELLY]

I just put the boat in the water today. It was idling fine in the driveway and was idling good at the ramp but then it just died. When I restarted it it kept on surging up and down. It would smooth out for a few seconds and then start surging again. I took it home and went through the carbs but they were perfectly clean. I'm thinking maybe this is an electrical issue? Its a 91 merc 35. Anyone have any pointers or trouble shooting techniques to get me started? Thanks