QIM, head over to Zekreet-UTM coordinates N 25° 29' 24.31'' E 50° 50' 40" (western peninsula) it is fairly deserted with soft sand, shallow waters for easy access for jet ski etc.
Also close to the old movie set if you fancy a walk around something different.
Or alternatively you could try 42km beach- Coordinates: 26°2'8"N 51°22'13"E
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http://www.everytrail.com/iframe2.php?trip_id=19520&width=415&height=300
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(before Fuwairit
http://www.qatarembassy.net/qatar%20gallery/Envirment/slides/Fuwairit%20... ) on the eastern coast above Al Khor (take north road and keep on trucking).
Or head up past the Losail Circuit (watch out for speed traps) and when you get to the Qatar Scientific Model Flying club turn right, at the aerodrome skirt round the left hand side (as you look at it) and head to the coast, there is a mangrove area however beaches to either side (steer clear of the 2 palaces though) and it is deserted with good waters.
The golden beach or the Algariah. Beautiful beach, unspoilt, water level never exceeds the threshold of you shoulders. Pass alkhor. Dont bother looking at the board.. Its 4 to 5 kms before port overlooking bahrain.
Right turn at the fork when you enter the road to the beach. The left takes you to a military base or something.