My wife and I face this same dilemma annually, and the solution is to just plan to pay some bag fees. I have never had them physically take a carryon bag from me because it was too heavy. They also have only made me pay the overweight fee twice in about 10 legs back and forth from GC to LC. Even if they do, it is only $0.50 per pound. So if you are 20 pounds over weight, you are still only talking about $20 total for both trips. Less than a single bag fee on most airlines, and well under the $75-200 overweight fees charged by most US airlines.
Here are a couple of tips for what has worked for us in the past. On the Otter from GC to LC and back, you can check two bags totaling 55#, and one carryon totaling 15#. However, most airlines coming to GC will charge you for each checked bag, but will let you carry a carryon and a "personal item". So here is what we have done in the past. We put all of our camera gear, batteries, chargers, portable hard drive for backing up dive photos, and small miscellaneous gear in a carryon sized pelican case. We put our BCs and fins in one large duffel and clothing and toiletries for both of us in another duffel. My wife puts both regulators and both masks along with a book and snacks in a regulator bag, and I put our small tablet, passports, kindles, and other miscellaneous travel stuff in a medium sized backpack. For the flight to GC, we check the two duffles, and my wife carries her reg bag on, and I carryon the pelican and the backpack is my "personal item". When we get to GC, we check the two duffles and the pelican case and wind up at about 115-120#s for the two of us. They may or may not charge us $5-10 in overweight fees. I have never had them weight my carryon bag at the GC airport, but mine usually weighs around 20-25# and my wife's weighs around #25 with both regs in it. When you leave LC, they will have you put all of your combined bags on the scale one at a time and then add it up to see if you (as a couple) are over the 140# total (2x55#+2x15#) and may or may not ask you to pay for the overage there.
Here are a few more tips. Don't pack a bunch of clothes. We know we will usually end up buying a shirt or two from the dive/gift shop. So pack 1-2 fewer than you need and buy them at the beginning of your stay. Also, the water this time of year is generally very warm (84-86F generally), and you may not need any exposure protection beyond a rash guard while underwater. The last 3 years that we have gone in September or early October, I have been diving in shorts and no shirt or shorts and a long sleeve rash guard. I carried a 2mm shorty for 2 of those years and never even got it wet. They only offer night dives 1-2 times per week, so if you go on one, just rent a light instead of bringing your own. Other than that, just realize that although the food is fabulous, the dining room is a VERY casual place, and shorts, t-shirts, and flip flops are what almost everybody will be wearing to meals. So again...don't pack so many clothes!
I hope that helps...now I have to get back to figuring out how I am going to pack for this year's trip in late October now that we will have 2 camera rigs instead of just one... I wonder if they would be willing to factor in the fact that we have lost over 100# between the two of us since February in whether they charge us more bag fees?