I don't think you've missed anything. Sometimes the titles are simply inspired by some phrase, which may only come at the end of the story. For example, I spent ages wondering why the hell "Walk on Water" was called that until one of the two protagonists said something like "being with you is like walking on water for me" in the last chapter. I think "Wet Sand" might also have a moment one day where we all say "Aaaaaah, that's why!".
Maybe it's the song by the RHCP?
Or maybe it's Wet Sanding were you remove the rough stuff ?
I think its probably cuz it's very hard to walk on wet sand and characters in this manhwa have very hard life to live and hard path or so I thought to myself
why is this story called wet sand?? like, im sorry if I'm just dumb or missed something, but I kist need to know!!!!┐(‘～`;)┌