Check the above water line thru-hulls, such as for the sinks, tank vents and bilge lines. On my boat, I had a similar problem when healed towards starboard. (Although the rate of leak was much less than your description.) The caulk seal on one of the thru-hulls for a fresh H2O vent, located only about 12 inches below the toe rail, had failed and the locking nut on the inside was very loose which opened up a nice gap. A way to check as already mentioned is to have someone blast around each thru-hole with a fresh water dock hose and see if water is entering. This can be difficult though if you have a void space between the hull and the interior liner. Water can enter through the poor seal at the hull and you might never really see it coming in. Creative detective skills must be employed! Also, poke around each thru-hull and twist etc. See if anything is lose.
Yeah ... the hull to deck joint also can be a weak link.
And as already mentioned