MIC: Is it illegal to leave the presence of your parent with alcohol still in your system?

I received an MIC on the Forth of July while on a boat with my ‘of age’ boyfriend. The officer and game warden were going from boat to boat. When they came to ours they did a routine life jacket check then proceeded to ask if there was any alcohol on the boat. My boyfriend replied yes, however there was no alcohol in sight at anytime. When I was asked by the sheriff if I had had anything to drink I replied yes that I had early in the day. I was honest with him and told him I was 20. He then gave me a breathalyzer and wrote me up an MIC. I was VERY respectful, yet, he wouldn’t let me explain the situation. To my defense, I had a few beers earlier that day with my parents and whole family at a cook out at my home. My mother drove me to meet my boyfriend at the lake. The only time I consumed an alcoholic beverage was in the presence of my parents. I have pled ‘not guilty’ and have a trail set before the judge. Is it illegal to leave the presence of parent with alcohol still in your system? What are the laws the judge may have under his belt to find a way around my innocence? Is there any information you may find important to help me win my case? Thank you in advance!!