91.117 a)
"Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h)."
The US airspace extends to 12 NM from the coast, so my understanding is that this regulation applies as long as you are within that area.
Not sure if this answers your question, but I hope it helps.