I don't know if 10,000 HOURS on scuba is possible. I know there are a few who have 10,000 DIVES. I probably have practiced/played clarinet well over that, but that's since 1963. Anyone out there figure they've done 10K hrs. underwater?
Considering diving isn't just about bottom time but includes planning, gear configuration, dive briefing, dive logging (or for those who don't log, a debrief: "did you see that bright yellow fish? what was that?"), training, and teaching I think there are quite a lot of people who have logged 10,000 hours. Especially those in the 1,000s of dives range.
Example, for a weekend trip (2 - 4 hours of bottom time over 1 - 2 days):
1 - 3 hours: researching dive locations, checking weather, and planning the trip.
1 hour: going through equipment and checking that it's ready for diving.
Spend much of the road (boat?) trip discussing the diving we will be doing with buddy.
30 minutes: setting up equipment and pre-dive checks.
5 minutes: on a dive briefing.
30 - 60+ minutes bottom time (depth dependent)
45 minute surface interval swapping tanks, briefing for next dive
rinse and repeat for number of dives on trip.
2 hours: logging dives over a meal and drinks, discussing the things we saw and did
4 hours: cleaning and putting away equipment for next time
countless hours on SB reading, learning, and discussing diving
That is all part of the hobby for me.