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. !