To answer the OP:
1. Wetsuit material that has all of the physical properties of Yamamoto 45 (used in freediving suits) but incompressible. I can stay comfortable for hours on California’s north coast… until hitting 40-60'. It would be so nice not to have to use drysuits for Scuba.
2. A small, reliable, and inexpensive gas chromatograph that could run a rebreather or monitor a compressor.
3. Reliable and affordable wireless acoustic communications systems
4. Acoustic homing devices small and inexpensive enough to augment compasses in dive computers. It would be OK if transmitters were also required for the boat, divers, and for marking locations.
5. Decompression computers based on bio-sensors rather than algorithms based on tissue averages, depth and time.
6. Vast Helium reserves that would drop the price to about 2 cents a cubic meter… in my back yard would be good!
7. A divers elevator/lift (becoming somewhat common in the UK) on every dive boat.
8. A decompression pill to make chambers and saturation diving systems obsolete would be nice.