I would have to say no. Some people remove all of this material. From what I've heard the reversion that everybody is scared of is more from the water coming out of the exhaust ( through the water jackets exititng the tips). Than water actually coming in from the tips. Thats why marine cams have atlest a 110 deg lobe seperation. If this seperation is not wide enough it can cause alot of vacuum in the cylinder as the piston is going down right after the exhaust stroke. Wich can suck the water, that is already running through the manifold, into the cylinders. As far as water coming in through the tips, if you came off of plane and immediatley went into reverse. The wall of water you back over could, possibly get high enough up the exhaust to cause a problem. Also you have to consider where are the tips? Mine are just below the waterline and I have eddie marine tips. I'vew never had a problem/ But, I don't come off of plane as I described. Anxious to hear what other people say.