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.
2005 X2, Viper Red, MCX, OJ TZ-X5 prop, PPass, rear 750 sacs, KGB, IBS, Bennett Wake Plate, custom wake shaper
(previous) 2001 X5, 1991 TriStar 190