Some beachside sites (don't know if you've tried them already - perhaps you could post a list to save repeating recommendations).
Camping Cote Vermeille at Port la Nouvelle, Les Vagues at Vendres Plage, Beauregard Est at Marseillan Plage, Beau Rivage at Meze, Le Tamaris at Frontignan Plage, Serignan Plage at Serignan, Le Boucanet at Grau du Roi - all these have access directly to the beach.
Have you tried Camping Neptune at Agde? It does have a pool, good sizes pitches, and is directly on to the Herault river. It isn't directly on the beach, but not that far to walk or cycle to the beaches at Grau d'Agde - and is also near to the Canal du Midi and the town of Agde itself (not Cap d'Agde of the naturist sites - but the old town which is much nicer).
It's just a thought if all the 'front-line' beach sites are full already.