This area has been in the process as being nominated as a UNESCO world heritage site mainly for its beauty and geological formations. It is actually two inland seas. The first still being 10-13 m deep and the inner one only 1-3 m. Both are connected by a small channel to the Gulf proper. No boats (except caostguard etc.) are allowed here so it is almost a marine reserve. Lots of dunes to test your driving and sabkha that can be tricky when wet. It is a beautiful location.

You can go with the various desert tour companies for around 200QR each person. 4x4 essential.

Typically you go down and have a few beers whilst looking over the water (only a few km) to Saudi...

There are a few good dive sites here.