Things to check ; Sterngland/Lazarette or other
deck hatches that may leak water running down the deck/Skin fitting not tight or if plastic - cracked under the backing nut - very common/cockpit drains or
cockpit lockers leak into bilge/engine
cooling water leaking somewhere. Try to ascertain if water enters only during sailing or if also during rain on moorings. Taste the water - may be
tanks leaking. Run a hose over the
deck while checking below. Good Luck. !