I fly fortnightly normally, (commute UK to middle east) I always carry some scuba gear, and have taken most of my scuba gear as carry on at some time or another. I have never had a problem at security or with the airlines I use. My flights are always International.
I love British Airways, their hand baggage allowance is two pieces, max 23 kg each, but limited to airline carry on dimensions. I always carry my regulators, computers and camera in hand baggage, normally I use a Transpac soft harness, but if I take a plate my backplate also goes in hand baggage, as do my reels, SMB, torches and other accessories, and on my last trip so did my wetsuit.
The only thing that goes in the hold are my fins and knife/cutters. A couple of times I have had questions about my umbilical canister light, but screw the head on, light it up and they have been fine.
Personally I'd never pack regulators or computers in the hold, I have seen plenty of damaged cases, even so called 'hard cases', and have seen the way they throw hold baggage around. I have never had a problem with hand baggage getting damaged. But mainly it is down to weight. If you can pack it in hand baggage I would do so,
Otherwise wetsuit, BC/Wing, fins and knives in the hold, everything else with me. - Phil.