I have through-the-transom headers (safer for my young son) on my boat. Last summer one developed a leak where the log bolts to the riser. The water that crept in through that leak did get sucked into the cylinder and rather quickly polluted the oil. I spent a lot of time and money (esp. $ on cases of oil!!!) trying to track down this problem. I have a rather large cam so the exhaust valve is not closed all of the way when the piston is on the first part of the intake stroke. The water from the exhaust is sucked in. My pistons are tight, the block was just machined and new pistons fitted last spring, yet the water got around. If what Ive described doesn't make sense, I'm sure someone here will point out the flaw in my descriptions. But that's the problem the way I understand it. The water in the exhaust will only be a problem with a large cam. Perhaps someone here knows more specifically what size cam will be a problem for you. They may need to know your engine type and size. Mine is a 460 Ford. G'luck-hope that helps.
Jason.