My lads enjoyed snorkelling at Sharm so you could check their hotels out. You want one with a beach and jetty to enable you to get to the coral easily such as the Conrad. There is activity in Sharm in the evenings for when you want to get out.

Alternatively something like the Aquafantasy in Turkey which gives unlimited free access to one of the best water parks in the med and if you get bored there is another one up the hill! Kusidasi is a dolmus ride away with plenty of lively bars in the evenings.

There are also good AIs in other Turkish resorts but I don't have personal experience.

Much further afield is Mexico, We had 2 weeks AI last year in the GBP Coba for about your budget so may be worth considering. There were evening trips out to the local Senor Frogs! There were several day trips educational and fun. The entertainment crew certainly entertained the 20 yr age bracket.