Here's a link to prior post(s) w. links to write-up, pics, etc of a possible source
Mainships & others are somewhat notorious for leaks at the rubrail / hull-deck joint
If you look at the linked pics you can appreciate why.
You should be able to confirm this as a source before tackling the repair by drying our - taping paper towels in suspected areas and using a hose to rinse down the rub rail area at sides / locations where it seems to be accumulating.
In the spirit of full disclosure - others have proposed simply caulking the top of the rub rail at the upper deck - I consider this a band-aid temporary fix that does not get at the root cause shown in the pics... it will work for awhile and require re-do after some time of after the hull flexes and breaks the exterior caulk loose.