At $30 one-way, it is a lot cheaper to check a second bag that to gamble with overcharges on a single bag. Last trip (June 21-July 1) my wife and I did that with room to spare.
That said, as of right now this is Delta's charge:
Travel to/from/within Mexico, Central America, South America and the Caribbean
75 USD/CAD* for bags weighing 51-70 lbs.
200 USD/CAD* for bags weighing 71-100lbs.
Under "Sports Equipment" it says that, excluding tanks, scuba equipment can be checked as "normal checked baggage."
Previously, American has had about the same level of charges. I am in the Washington DC area, so Delta and American are my main options. For me, Continental has been out of the question for several reasons for a long time.