With plywood construction you have the great benefit of only having a few seams to consider when looking for a leak. The keel-bottom seam is the likeliest villain, as it is usually under the greatest stress, followed by the chine seams. You could try allowing the water to build up a little, and when the boast dries out at low tide, get under and see where the water is dripping from. Where it goes in, it will also come out. This would not work for a chine seam, as the level of water needed inside the boat to show a leak will cause too much stress on the hull, making things worse.
Peter.