You are correct - there is no real way that you can be sure your stuff will stay secure. Sad but true.
We accept this fact, and carry-on the gear we would be miserable without - mask, reg & computer, and custom wetsuit - and live with the risks.
There are things you can do; avoiding airports like San Juan, using regular luggage bags (not scuba luggage that screams out "valuable stuff inside"), packing so dirty clothes are on top, etc. We use colored zipties so if they are cut open we know it immediately.
The "TSA locks" are a joke; the thieves have copies of the keys, and if the TSA gets backed up they just cut off the locks anyway. Save your money.
Yes, it's a brutal travel world out there, but with some care and the right frame of mind, worth it.
All the best, James