It is the time of year where divers will be out frequently in the Boston Harbor area enjoying their sport. This means boaters must be careful and observant of them and avoid the areas they are diving in. Massachusetts law requires divers display a diving flag (seen below), and stay within 100 feet of that flag. At the same time, boaters must stay at least 100 feet away from these flags, and if they must approach, they must keep speeds under 3 miles per hour.
Diving Flag. If you see these flag, this is the indicator that recreational divers are down. Stay at least 100 feet away from this area.
Alpha Flag. This is less common, but is a sign that divers are down around a vessel with restricted maneuvering capability.
Thankfully, the incidence of boater-diver collisions has been historically low in Boston Harbor. However, this is still a very important safety consideration, as the outcome of a collision could be catastrophic. There are many times when I have seen a boater get too close to a diver down flag. The risk is increased in rough seas and large swells, as a flag could temporarily disappear from a boaters view under a large swell.
There are several dive companies and many individual divers that frequently go out in Boston Harbor. I find that the places they most often dive are: 1) West of Graves Light, 2) Around half tide rock and Calf Island, 3) on top of the ledges outside the harbor entrance, including Martin Ledge, Thieves ledge, and Boston Ledge. When in there areas, be particularly careful, and look out for diver flags as you pass through.
Seen below are some boaters parked along the inner side of Calf Island in the outer Boston Harbor Islands. In these areas the ocean floor is deep and jagged, making it very interesting for divers to explore.
Divers at the end of Outer Brewster Island, with a symbol well displayed:
Another popular spot is Egg Rock, up near Nahant: