Use "on" because it implies that you were there on the sand looking for shells.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Anonymous'I was looking for shells at the beach"is this sentence correct?Yes if you are speaking of the location (i.e., not in the city)—or 'on' if you are speaking of the sand and not the sand dunes.