This app was mentioned in 2 comments, with an average of 2.50 upvotes
Ambient Mixer - roll your own background sounds, plus a directory of pre-made stuff. Personally, I like "sailing": waves, seagulls, wind, ropes creaking. Better than waves alone.
there's a free version too! https://play.google.com/store/apps/details?id=air.com.ambientmixerfree&hl=en