I caulked the top and bottom of the rail when I first refit my V25. Water still got in and the easiest way to get out was through the bolts that hold the deck and hull together, The water followed the bolts to the inside of the boat
. It was especially bad when the boat was on the trailer and I had the bow up so the cockpit would drain. The water ran to the back and came in along the hull/deck joint along the transom and puddled on the floor there. I dug the caulk out on the bottom side in the rear corners to open up a water channel. It helped some. I actually found that keeping the bow down as far as I can when on the trailer stopped all water ingress along the rail. I get no water coming in along the rail when the boat is in the water. With all that said, it is a good idea to caulk the rail, if you sail a lot with the rail in the water
. I have read of many Mac sailors complaining that they get a lot of water ingress along the hull/deck joint when heeled over.