My strong hunch is rainwater. Remove your engine cover and look for a streak of grey dirt residue left by infiltrating rainwater. It will give you an idea where to start looking.
With someone hosing the deck crawl head first on your back into the each of the quarter berths. With a strong flashlight, closely check the bolts on each of the stanchion bases. I'd also look for leakage at the seal between the gaiter for the bilge pump and the lazarette, the engine controls, the tangs for the backstay bridle, the padeyes for the spinnaker sheet turning blocks, and the mounting hardware for the winch handle pockets.
I expect that the limber holes that should be draining this water into the bilge are plugged with the residue from the rain. I found the limber hole by at the mast step was clogged with this gunk. I'm amazed how much of this I remove when periodically clean this hole with a machinist's pick.
