From PortlandPaddle.net:
Fort Gorges, a Civil-War-era fort in the middle of Casco Bay, is one of the most fascinating places to explore in the area and it’s only accessible by small boat! We’ll start off this guided trip with a paddle into Portland harbor and then make the crossing out to the Fort, where we’ll land on a beach and allow plenty of time for exploration. Your knowledgeable guide will tell you all about the Fort’s storied history.
FMI and to purchase tickets: https://www.portlandpaddle.net/tours/fort-gorges-sea-kayak-trip