Penobscot Bay is Maine's largest bay, stretching approximately 20 miles from Whitehead Island to Isle Au Haut. The bay is characterized by numerous small islands, rocky shorelines, and coastal villages, forming a central section of the Maine Island Trail. Paddling here involves navigating formidable open-water crossings between islands, though protected waters can be found near harbors like Belfast, Searsport, and Stockton Springs. The bay experiences significant tidal currents, particularly in thoroughfares like the Fox Island Thoroughfare, and paddlers must plan crossings for slack tide or periods with minimal wind to avoid challenging conditions. Water temperatures are cold year-round, necessitating appropriate safety gear and respect for the marine environment.

Public access points are numerous but vary in capacity. Key launch sites include the public boat ramp at Harbor Park in Camden, Steamboat Landing in Camden, the boat ramp at Snow Marine Park in Rockland (which may charge a fee), the Port Clyde Boat Ramp at the end of Route 131, and the Tenants Harbor Town Landing. Other access points exist in South Thomaston, Lincolnville, and Castine. Parking is often limited, especially at popular ramps like Port Clyde when the Monhegan Island ferry is operating. Amenities at launches range from basic boat ramps to facilities with restrooms, docks, and floats.

The bay sees considerable boat traffic, including lobster boats and larger vessels in main channels, so paddlers must maintain vigilance. Wildlife sightings may include harbor seals, grey seals, harbor porpoises, bald eagles, ospreys, common loons, and various seabirds. The scenery is exceptional, featuring historic lighthouses like the Rockland Breakwater Light and Curtis Island Light, as well as the undeveloped Sears Island. Guided tours are available from outfitters in Castine, Camden, and Rockland, offering instruction and route planning for those less experienced with the bay's exposed crossings and tidal forces. The best time for paddling is generally during the summer months from June through August.