Assuming it is not coming in through leaky fittings, then the favourite would be at the seams between the bottom panels and the keel/hog or chines. Take up all the floorboards and bunk tops so that you can see the seams and watch for where the water seeps in.

As to repairs it depends on how these seams were made. Sometimes they are glued in which case the glue line may have failed. Or they may be screwed down and bedded in a flexible sealer. In both cases the failure may be accompanied by local rot/delamination and or screws corroding or pulling out. Best to get it fixed quickly because it is likely to deteriorate at an alarmimg rate.