Although it's dangerous and rugged, for about two hours when the tide is out, the hearty and adventuresome visitor can climb the steep, slippery metal ladders, walk on the ocean floor. Then, across two intermediate islands--connected by a short, wooden bridge--and once again down a steep ladder to cross a rocky, wet, seaweed-covered intertidal zone, and then finally out to the lighthouse itself!
DANGER!--TAKE NO RISKS & DO NOT LINGER! If you become stranded on the islands by the tide, WAIT FOR RESCUE. Even former keepers of this lighthouse have lost their lives by misjudging the STRONG, FRIGID, FAST-RISING tidal currents, and TIDE-PRESSURIZED UNSTABLE PEBBLE OCEAN FLOOR, while attempting to make this crossing. (During a summer in the 1990s, two visitors attempted to swim across this passage. One made it across, but the other was swept away by the current. After a rescue by boat, both had been stricken with hypothermia, were rushed to the hospital -- and luckily, survived.) [end quote]
Tides in the lower Bay are 20-24 feet. Check an online tide chart so that you time your visit at the turn of the tide, usually most effective to see the lowest tide and then return to see higher tides unless you want to wait the full period (about 6 hours) for the hight tide. (Tide charts give predictions, not certainties.)
If you go onto Campobello, you are, of course, going to Canada (N.B.) and may need proof of U. S. citizenship (passport or license + birth certificate) to return without hassle.
Harbour Head lighthouse is a beautiful spot on its own, by the way.