500N - The CTBTO obviously set up parameters for background or white noise for the likes of waves, and can relegate the noise to the area of no consequence.
I would expect a ship would have a different noise profile to an aircraft hitting the water, and that the major source of noise from ships would be engines operating.
Ships hulls crashing down into troughs would be only be an event that happens in extremely heavy seas, and as the CTBTO can pick up the direction where sound is coming from, this sound would normally only be heard from the direction of the Southern Ocean, and during sizeable storms.