My visit to Cartaya was to party in New Year's eve. I must confess that I almost skipped town, because the hotel was in El Rompido, and the following day was used in Seville. A nightly stop, and it was all.
We visited El Rompido because... we were there - we attended New Year's eve party in the hotel, and being in the area we had a look at the place, before the planed visit to Seville.
Only a general idea, but... does it deserve more?
El Rompido beach
We've been there in winter, so we couldn't get a clear idea about the beach in summer, however, we could see that the beach stays almost one kilometer from the open sea, sheltered by a long and large bank of sand.
The water is obviously calm (maybe hot),ideal for children, and the long protected natural marina full of small boats.
It seems to be a calm place, good for families with small children.