If you book your trip, normally the Tour Operator or the Dive Center will make for you the organisation of the taxi for a reasonable price. If you go diving in Marsa Alam, be aware, that beside the Hotel Resort you will find only desert and no infrastructure. The prices are generally more high than north, specially at the moment where the Tour Operators push the South of the Red Sea. If you go a little bit north - like Safaga - the Reefs are not so busy as Southern Red Sea for the moment.
..... why not choose Safaga (which is very quiet but with some infrastructure at least, some shops and some Pub's to go drink a beer outside the Hotel). Safaga offers daily diving with the boat for EUR 190 (10 boat dives) 2 dives every day. Depending where you go in Marsa Alam, there will be offered shore diving and boat diving with long ways to the dive spots - depending where you go also with a transfer to the next marina where you will find the daily boat. Depending where the center is located and depending if the center has own boat-marina or not.
Also you can find more info in
Marsa Alam dive Guide