We flew Sacramento, CA to Miami, then Miami to St. Maarten and return flights, and checked our bags (both of us managed to pack our clothes in with our dive gear), both had our one personal bag and one carry on (mine had my photo & video gear & housing, my husband carried our regulators). I had gone to the airport before to make sure my carry on would fit in the new sizing holders they had ... yup. Although one did try to take it away, and then I looked pointedly at the person in front of me who had 4 carry ons and I pointed to that person, and the flight attendant shrugged and let me on.
We only had one airline, on a prior trip (Dec. 2000), charge us -- and that was our own fault as it weighed 92 pounds (of course, that was the trip that we both packed our gear in one bag). We then redistributed things and now all our bags are under the 70 lb limit that seems to be almost normal for most airlines now.
We also packed a large duffle, empty, in the dive bag to pack our clothes in should we do that last dive too late for our gear to dry and have some damp gear for the return trip home ... which we didn't need, but we packed it with dirty clothes. It was nice to have that extra bag so we didn't have to stuff things in the others.
Enjoy your travels.
Margaret