If the hole was in the top of a rail in an area that goes into the water, as the rails cool, they suck water in but it can't run out. Water is amazing stuff - gets into places you wouldn't believe until you have seen it yourself. When I first fitted my trim tabs, I used enough sika to near on build a boat, prepped all the surfaces and the covered the screws in sealant prior to screwing them in and yet still had one side that leaked. I recently had to replace a heap of mild steel fresh water pipe in one of our vessels due to internal corrosion causing blockages. Re did it in stainless using stick. I had a few leaks (par for the course - I'm no gun with the welding) but the amazing thing was just how much weld the water could track through if there was the smallest bit of slag in it. I had several where I basically had to run the grinder through a weld that looked perfect except for the tiniest black dot to totally get rid of the slag so the weld would seal. If I just welded over the top, it would surface again and still leak.