So.... has anyone ever used scuba gear to set

To set traps in beaver dens? Yes , thats right diving under water to set the entrance to a beaver hut.
I have been hired to trap a large lake in a very upscale subdivision. I counted at least 4 active bank huts that are on the edge of deep water. The only way to trap them is to use lured sets on the bank or set the dens. I'm sure I could pick them off one or two at a time but with the cost of gas I may try it to speed things up.
Im just not sure how its gonna go down when Im placing a 330 in a hole and a beaver decides to leave, and hits the trap a foot from my face 6 ft down ?? Ive had it happen while standing in waist deep water and that was bad enough...

330s are a strong trap. Make sure you wear a B.C. so hopefully you will have a free hand to get you to the surface if theres an accident. I realy wouldnt recomend it though. Too muddy to see the small stick hitting a trigger, etc.

How bout LONG very long stablizers that can be set from land/boat

I can't believe you asked that question. I'd forgotten all about my scuba diving; looking for deep muskrat holes on a lake. That was alot of work for one little muskrat but the water was crystal clear and the water temperature was great. A lot more fun than salt water. ( Probably because I can't keep my mouth shut under water either )

If you get trappers world theres a trap on the front page it's great for sets like deep runs , it's a comstock beaver trap , you just lay out in the run , there no way for beaver to fire it if he's not in it , no stakes , just pitch out there like a bag of rocks , should be a feed hole around to , that's also a good spot

Just use the tee-pee set for deep water entrance.

I think it might be a bit unsanitary to be diving in beaver-infested water. Might get giardia.

When you dive, you always get a bunch of that water in your mouth, can't be helped.