Swim platform noise
What you describe reminds me of the noise that comes from water hitting the underside of a swim platform on some powerboats. Depending on the gap between the underside of the platform and the water some can really splash the water around. The device(s) your looking for work to displace the water, thereby reducing the noise. I've read reports reflecting varying degrees of satisfaction with the device. Before you spend money you could try a couple of things. The first would be, if possible, to dock opposite to how you are now. Hopefully the boat will ride differently against the prevailing tides and winds. The other would be to try to jury rig some fenders or even some of those cheap orange lifevests to the area where you believe the noise to be coming from. Anything that would prevent the water from slapping that area of the boat and not cause additional noise itself should work. With any luck you'll be able to figure it out and sleep better. For what it's worth, all the noise from the water is like music to my ears, I sleep like a baby.