Hi Jibjab,
I just booked a trip to Dahab for April (wife and kids ages 12 and 14). From the research I've done, it looks there is decent snorkeing in various areas of Dahab, from various areas on the shore close to town or outside of town. It also sounds like there are many, many local tour operators that will take you to more remote areas to snorkel - via 4x4, camel or boat. A large amount of the diving in Dahab is shore diving, so I have to assume it would be quite easy to tag along on dive outings where the snorkeling would also be very good.
It also definitely sounds like Sharm is not the place for you based on the vibe there....bigger mega-type resorts, more crowded. Dahab is much more suited to individual travelers who want a more laid back vibe.
Dahab also seems to be at least slightly better situated if you are interested in desert adventures, such as hiking Mt. Sinai, overnight trips in the desert etc.
All that said, this is my first trip there....so all of my info is second hand.
One place to look for additional info or to ask questions if you not getting adequate answers is the Dahab forum on trip advisor...give it a try:
Dahab Forum - TripAdvisor
Like any web-site, you to take the opinions with a grain of salt (not knowing anything about the posters), but there is quite a bit of info about snorkeling in Dahab.
Another option could be southern Egypt - south of Marsa Alam. There are several small dive lodges in that area that are very relaxed and although they cater mainly to divers, the snorkeling is pretty good as well. We stayed with Red Sea Diving Safari in Oct 2007 and had a fantastic time (they have 3 locations down there - we were at Nakari village). These places are pretty basic and VERY quiet....the folks there are there to dive and snorkel. At Nakari they had RIBs/Zodiacs that would drop you off at various spots aong the coastline and then you could snorkel back or they would come and pick you up. We also did 3-4 trips to Dolphin House reef, where the snorkeling was also very good and where my wife/kids were surrounded by dolphins multiple times out there, including mother/baby dolphins. They also often offered "truck dives/snorkels", where they would load everyone and the gear into a truck and spend most of the day diving/snorkeling at a remote site. On one of these we did some great snorkeling with several large sea turtles - fantastic.
Here is the web site for Red Sea Diving Safari:
http://www.redsea-divingsafari.com/
The advantage of heading further south is that water temps may be slightly warmer vs. the Sinai.
Hope this helps - and happy travels
Chris