Next summer we would like to do an early summer vacation in Maine. We are interested in hiking, bike riding, boating and swimming. We would like to visit Arcadia, eat lots of lobster and visit area colleges. A large Airbnb would be great. Husband went to Bowdoin so that is a must. Kids age 16,14 and 11. There seems to be so many lakes and areas but I have been frustrated trying to narrow it down.

If you're looking for lakeside then look at the region around Hope, Alford Lake, Hobbs Pond. Then you'd have easy trips to oceanside Camden and Rockland for lots of dining options.
The area is about 90 mins drive to Acadia and 90 mins in the other direction to Bowdoin.

I like Clark's suggestion although I'm not familiar with the lakes. Rockland and Brunswick areas are some of our favorites. Visit Bowdoin on a Friday when the Brunswick farmers market is open. Maybe add a visit to a hiking area in the Harpswells. You can download a guide to conservation areas. With all the hiking and biking opportunities in Acadia you might want to split your visit between two areas. Some air b&b places will do less than a week rental especially if before July 4. Bath is just north of Brunswick. The Maritime Museum has great boat trips to see lighthouses.

Wondering why you mentioned 'lakes' for a trip to Maine. While there is nothing wrong with the lakes in Maine, in my opinion they are not all that unique compared to lakes in other places. On the other hand the coast is incredibly beautiful. I don't know where you are from but when I travel I try to see whats most interesting and different about the place I am going compared to places closer to home. In the case of Maine that would be the coast. Not to mention that you'll find more lobster shacks close to the coast. And Bowdoin is near the coast. And the area near Bowdoin, between Portland and Camden/Rockland is one of the best parts of the whole coast.

Are you looking for one place for a base from which you can do day trips or are you willing to move around? I'd probably do a few days up by Acadia itself, then move south and stay around Boothbay Harbor. But if you want just one place for the whole time, Camden (or somewhere close) would be perfect. Still close enough (less than 2 hours) to get to both Acadia and Portland.

If swimming is a priority, plan to go in August when the water temperatures might get above the 60s. Finding lobster near the coast is easy. Drive times will be longer along the coast roads than given by the mapping sites.

YOu can't really swim in Maine without a wetsuit. Mid-August is the warmest sea temp,of course, and right now it is only 69F at one of the most southern beaches (Old Orchard and Kennebunkport). But the husband should know that if he went to college there.

I think the Bristol/Bath area would be a good base for day trips and perhaps more accessible than Boothbay Harbor (meaning to get on the road to go somewhere).

I don't know anything about lakes in Maine, just have been on the coast.

Mid coast where I go several times a year there are plenty of place to swim comfortably in sheltered bays/inlets without a wetsuit. Open ocean is of course colder, but much of Maine coast is not just open ocean.

I've been to 11 or 12 Maine coastal beaches and I've never even seen someone in a wetsuit.

This lakeside spot is about halfway between camden and Acadia National Park....about 45 mins drive to each
https://www.airbnb.com/rooms/19600470

I have been swimming in Maine for over seventy years and the water is cold, but not always unbearable especially in August along the southern beaches. When I Was a kid my mother made us come out and warm up when our lips were blue.

if you are used to swimming in the Gulf of Mexico you might want a wet suit.

Over the years you figure out that Maine beaches are most comfortable in the late afternoon when low tide was late morning and the sun warmed the sand before the tide came back in over it. That said I also stepped into the ocean in Tenant’s Harbor one June and my ankles ached from the cold in ten seconds.