According to the St. Clair County Marine Patrol website, .10 for a 200 lbs. person it 6-7 drinks in a 2 hour period. Thats a glass of wine, a shot of 80 proof whatever, or 12 oz beer.
What makes things worse, if you get a DUI on the water your driver license gets effected and you car insurance rates too. That was an expensive party.