lap against (something)
Of water, to break, beat, or splash gently against something, as due to a current or tide. I love sleeping out on my uncle's boat at night, with the waves lapping against the hull.
lap (up) against something
[for waves] to splash gently against something. The waves lapped up against the shore softly. The waves lapped against the side of the boat all night long, and I couldn't sleep.