I used to live on Stratford Ct, which is the street that runs parallel to the water in Del Mar. 15th's a mellow break, you'll be fine. Unless there's a rip-current that pulls the sand off the rocks, it's a sand bottom and acts like a beach break. Underneath the loose sand is the same sandstone that the cliffs are made of. In other words it's not sharp, but it is rock. Go farther south towards Del Mar Heights Rd. and there's sandstone interspersed with harder rocks, hence the "Del Mar Reefs".