Copied from the United web site - baggage policy for sports equipment. I don't see the 70 lb limit mentioned there, but I know I have before. Not sure if it was with United or another airline, but several years ago they did paw through my dive bag to make sure there was no other stuff in there.
"United accepts one dive bag containing equipment used in the scuba-diving sport as checked baggage. Scuba equipment in addition to the baggage allowance will be assessed at the current excess baggage charge for a single piece, whether or not it is presented as a single piece.
Dive bags weighing more than 50 pounds (23 kg) that contain other items in addition to or in place of appropriate dive equipment will be subject to the applicable overweight checked baggage service charge.
Dive bags measuring greater than 62 linear inches (158 cm) that contain other items in addition to or in place of dive equipment will be subject to the applicable oversize charge.
First or second checked bag service charges may apply.
An empty dive tank or up to three rebreather tanks will not be included in determining the baggage allowance and will be subject to a service charge of $100 (each way) for travel between the U.S., Canada, Puerto Rico and the U.S. Virgin Islands. A service charge of $200 (each way) applies for all other travel. The empty dive/rebreather tank must have the regulator valve completely disconnected from the tank. The tank must not be sealed (i.e., the tank has an open end). The tank must have an opening to allow for a visual inspection by a TSA Security Screener.
Note for rebreather equipment: Soda lime that is 4% Sodium Hydroxide or less will be accepted in checked baggage. Soda lime that is 4.1% Sodium Hydroxide or more will not be accepted in checked baggage.
An item of scuba equipment consists of one empty scuba tank, up to three rebreather tanks, or one dive bag containing equipment used in the scuba-diving sport. The empty scuba/rebreather tank and a separate dive bag containing scuba equipment will be charged as two separate pieces.
United is not liable for damage to scuba diving equipment."