Flying with scuba gear

I just got off the phone with a United Airlines reservation agent. I
told her that I had one bag that was going to be over the 50 pound
limit for checked luggage, and I wanted to know how much extra it was
going to cost me to check it in. She said it's going to cost me an
extra $100 for this one bag! On United's web site it says overweight
bags costs $25 extra. So I told her that, and she says it's $100
because I'm going to Mexico. Why the hell does it cost 4 times as much
to fly an overweight bag to Mexico compared to everywhere else in the
world?

Also, she says that I can't lock the bag! WTF is up with that? I can
understand them wanting to verify the contents of the bag, but for her
to insist that I leave my property open to thieving baggage handlers is
simply outragious. Can't they just x-ray the damn thing? How do you
guys fly with your scuba gear?