Similar conditions in eastern Casco Bay. The mats usually float on the surface and don't bother a sailboat's intakes or propeller. When combined with other flotsam, however, the masses can lie below the surface. We recently picked up a big wad of rockweed that was tangled up in a short length of pot warp--had to dive to clear the prop. We were studiously avoiding lobster pots but had no warning of the mass lurking below the surface. In a separate incident, a remarkably long strand of weed was sucked into the head's raw water intake, disabling the pump itself--had to disassemble the entire pump and intake line. Fortunately, nothing broke. (If your head suddenly loses suction, you might have encountered a similar problem.)