When you take a walk at the shore, do you spend much time looking down at the sand? Probably not; looking down for shells maybe, but not AT the sand. Sand sculptors do.
It’s all about the sand. Look at it. Pick it up; touch it. Is it large granules? Does it have a lot of shell or pebble bits in it? That kind of sand won’t compact or carve well. Keep walking. Go past the areas of shore that has lots of shells, bits of seaweed, or large grains of sand. Look around some more for that area of fine-grained sand without much debris in it. That’s what you want to use. It may be dark or light in color. Doesn’t matter. That’s the good stuff.
While sand can be different at different shores or beaches, the quality of sand can also be different along the same beach or shoreline. It can be different within feet on any section of that beach. If you’re lucky, there will be good sand above the tide line. If not, find your good sand and work as the tide goes out. When your sculpture washes away with the next high tide, that’s a good reason to go back the next day with a new design to carve!