Acadia is deadly silent until the end of June when school is out for the summer. Every time i have gone there before july, the campsites at seawall have only been about 10 % full. No real need to reserve a tentsite, just make sure that there is a campground that will be open at that time. My favorite hike there was an unusually warm may day where i hiked in the morning and then had sand beach almost to myself, minus about 20 other people for the rest of the day.