It may be late to write in appreciation for such a wonderful Fourth of July celebration, but, I really enjoyed the fireworks and there's no better place then the lake. I'm looking forward to next year, realizing it could get even better.
With that said, I have a few suggestions. First, regarding swimming, a sandy shore that gradually slopes into the deep would be more family-friendly. Toddlers love sitting and playing in water and exploring and parents are more comfortable when their children have a little standing room.
Next, with no lifeguards on duty, people are already swimming at their own risk. Why not allow them to go the distance? It's a small lake and thousands of people could swim across it and back, or float and turn around at their own discretion.
Also, I personally like the idea of carnival rides on the lake on the Fourth of July, swim and ride! To make it a full day, you can allow vendors for the event. The pie ladies could sell their delicious pies to famished swimmers.
I hope that these ideas are not frowned upon with the attitude that you are pulling people away from the square. Who knows, more people may get out earlier in the day if it's like one big swimming party!