I've stayed in the Hyatt Regency in Taba and loved every minute of it. From what I have heard the Marriott is a great hotel, however as people have said there is nothing to do outside of the hotel complexes. The entertainment at night is virtually non-existant.
Snorkelling is great, particularly in front of the Hyatt. Hotel guests from any of the hotels can use each others reefs, so no problem there.
Diving is done through Waterworld, which although isn't the cheapest in the Sinai, its still of a high service standard. The diving itself is reasonable, nothing spectacular like you get further south. However, most of the reefs are pristine and if you are into the smaller and more interesting critters its a great place to go.
The reef in front of the Hyatt has been known to have upto 8 turtles in residence, especially towards late afternoon. A pod of four dolphins also used to visit quite regularly. The time I was their (July) they also had a young whale shark (about 6m) feeding in the main channel in front of the hotels.
If you fancy a bit of night life then just dive into a cab and go to the Taba border, Eilat is easy to get to and is a good crack at night. Parties at Dolphin reef are usually pretty good.
Some reports do state that the Taba trots are rampant. However if you follow the usual advice and keep your hygiene of a good standard then this shouldn't be a problem, I never got them.
I have pics of the Hyatt as well as the fish from the hotel reefs if you would like to look
http://www.pbase.com/benzonar/underwater