Here's my list and never had a problem or overweight:
LL Bean Rolling Duffel (Checked)
BC
Fins, mask, booties, 1.5mil vest
Mesh Bag
Shave Kit (all the normal personal+ Basic First Aid + Pills(Headache, pain, sinus, etc))
2 Swim Trucks
4 or 5 Tshirts
3 Shorts
1 Drawers (for return trip or nice dinner)
1 Collared Shirt (for return trip or Nice Dinner)
Beach Towel, Clothes Pins
1 1lb bag (unopened) Trailmix
1 hat
1 Leatherman tool (usually in backpack but TSA rules and all)
Backpack (carry on) (I prefer to stow under seat instead of overhead so nothing huge here)
Laptop
Gopro, etc (softside bag)
Regs, Computer (softside Bag)
Tablet (for reading and crosswords)
Everyday stuff (30' Paracord, Flashlight, zipties, bandanas, various straps, Sharpie, Pen, alittle bit of TP in a ziploc, magnifying glass, compass,etc)
I wear a nice shirt going (for a total of 2) that can I can re wear if we're going out.
I like having camera and regs in softsides so they can be dumped in xray tray then repacked easily.
Flipflops for easy on/off through security.
HAVE YOUR PASSPORT/BOARDING PASS ACCESSIBLE AT ALL TIMES!
This seems to work well for me. I hate getting behind someone that doesn't seem to know whats going on in the airport...
Also, I'm never even close to weigh limits (38-42lbs) so sometimes traveling companions dump stuff in my bag.