You can buy tickets at their gift shop at Sea Dawg Gift Shop at 47 West St. or at their boat location at 119 Eden St. (Atlantic Oceanside Hotel). The Atlantic Oceanside Hotel is just over a mile out Route 3 so you'll probably want to drive. There's plenty of free parking but realize that it's at the top of a pretty good hill. The free Island Explorer shuttle bus stops there as well so that's another choice.
The Acadia is a nice boat with upper and lower decks as well as an enclosed heated cabin. Dress warmly even if it's warm on land. We chose to sit on the lower deck as we boarded and we were almost alone for 10 minutes or so until those on the upper deck started coming back down where it was warmer. There were binoculars for rent for $3 and a modest snack bar.
There were 2 naturalists on board one of whom (Gabby) spoke on a speaker system. The presentation was easy to hear and very informative.
Calling this a Sightseeing/Nature Cruise is very appropriate as we saw lots of wildlife. We saw countless ducks, many seals and 7 (yes, seven) bald eagles.
As well as wildlife we got a different view of many of the features of Acadia National Park such as Thunder Hole and Sand Beach. We also sailed by Egg Rock Lighthouse.
Acadian Boat Tours offers discounts for seniors so be sure and ask for yours.
Their web site is here and the TripAdvisor reviews are here.