Echo Lake is a great place to cool off and spend a relaxing and fun afternoon.
The water is warm(ish)...for Maine anyway!..., it is clean, shallow and there is a lifeguard on duty from Memorial Day to Labor Day. You can spread a blanket out on the beach (there was only 1 picnic table), build a sandcastle, or throw a ball or frisbee around.
There is a boardwalk path that leads from the paved sidewalk down from the parking lot to the water. There are bathrooms and a separate building with changing rooms. There is an outdoor spicket to rinse sand off of your feet before heading back to the car.
Beware the over friendly ducks. They walk right up to you looking for food and will check out your bags.
My kids were having so much fun they didn't want to leave. If we didn't have reservations for another activity we would have spent a much longer time there.