Just ask for Ras Al Aktar/ADNOC beach or drive along the Corniche road all the way to the end in eastely direction. You will see a huge construction site which you simply need to follow.
To enter and exit the water you must climb down a small breakwater or paddle around it(2 min max).
Overview
Atmosphere
Adnoc Beach has a very relaxed atmosphere. The water is used by wind surfers and kite surfers which are all very friendly. It is not the greatest spot, but if you are in the citty, and want a surf, ADNOC is the place... But don't expect too much
General
ADNOC can be nice, but the distance between the waves can also be very short, so it is a bit of a pain to paddle back out.