I had the same problem with my '91 TriStar. Like a previous poster suggested, I rotated the vent towards the stern. Also, after comparing it with some other vents, I realize that the lower lip on the vent was broken off which also allowed the splashing water easier access. For good measure I scrunched up a small piece of rag and stuck it in the vent -- tight enough so it wouldn't fall out but not so tight it prevented air from passing. It wasn't noticeable unless you looked directly at it.
"For every complex problem there is an answer that is clear, simple, and wrong." (Henry Louis Mencken)
2005 X2, Viper Red, MCX, Acme 1285, PPass, rear 750 sacs, KGB, IBS, Bennett Wake Plate
(previous) 2001 X5, 1991 TriStar 190