The Turtle Hospital in Marathon rehabilitates injured sea turtles and runs guided educational tours. It is more rescue facility than aquarium, which is exactly why it is worth visiting: the stories are about boat strikes, fishing line, disease, and the hard work of returning animals to the water.
Tours usually need reservations, especially in winter and school-holiday periods. It is a good stop for families, but adults interested in the marine environment will get plenty from it too.