This was an awesome discovery for my boyfriend and I, because we didn't know it existed! We were lucky enough to be there at low tide. To find it, go to Schooner Head Overlook (there are signs for that, just not the cave itself). There's a short trail leading down the hill to the coast, right at the lighthouse. If you go at low tide, you'll see it near the bottom.
Acadia is an amazing park. I've been four times in the last three years, and I'll be going there for years to come. There's a bit of something for everyone here. Taking a drive around Park Loop Road is a convenient way to get to all the biggest sights, and you can even drive right to the top of Cadillac Mountain. The views are stunning all the way up to the top. There are hiking trails of varying difficulties. There are some hidden gems too - we stumbled across Anemone Cave (which was an exciting discovery since there aren't any signs for it and it has to be low tide to even be able to see it), and the western side of the park is a quieter retreat if the eastern side is too busy for you. I'm proud to live in a state with such a beautiful place as this park.
A couple of side notes - don't go during the middle of summer, do go in the beginning or towards the end of summer. It does get crowded in that peak summer time. Also, if you're coming from the southeastern side of Maine, I'd highly suggest skipping the highway for Rt 1. It's a bit of a slower drive, but it follows mostly along the coast and it's a scenic drive. Stop and visit (Maine's) Fort Knox for the history, and the Penobscot Narrows Bridge for the 446 ft high view!