Sometimes a stiff wire (electrical single wire or larger 3 wire romex cable) bent into a hook can drag lost items out from under.
Sticky tape (metal tape for sealing home duct work is very sticky) on the end of a wire has worked.
Put the rail in the water on a fast port or starboard tack or sail straight into an inlet with large steep waves that pitches the boat fore and aft and find lost items 'out from under' waiting to be grabbed up, pouring some water into the space on an even keel may facilitate washing items 'out from under' when the deck is tilted.