To be frank, if you're exploring Maine and not getting a taste of life on the water, you're doing it all wrong. But this outfit, which provides sea kayak and SUP tours around Casco Bay, will set you right. Some trips head all the way to Fort Gorges, a granite thumb of fortifications that seems to float on the bay.
Also rents kayaks (single/tandem half day $40/55) if you want to explore on your own. Has reduced hours in May, September and October; call ahead for details.