Jellyfish not a major problem if your porperly kitted out (yes that means a lycra suit in the summer so consider tonning those abs....

Diving here is a combo or shore diving and for the more qualified boat diving which is predominately wreck (10-50+ m ). Some offshore islands allow shallow diving.

There are very few true reefs in Qatar, those that exist are to close to sea border for people to venture there without permission and a "guide"

Viz ranges from 10-15 m on good days to 1-2 m. Site dependant aswell.

Boat dives cost 200-300 QR in a club scenario, shore diving is free... if you can get there.