That's likely the problem. The previous owner of my TriStar told me that he would shove a tiny wad of rag into the vent to stop water from getting in. I didn't believe him . . . but I should have.
My first couple of outings with the boat, sure enough, I got water in the tank. I started plugging the vent like he suggested and then it was OK. As I mentioned above, eventually I turned it so it was facing the stern and then all was well.
Yes, there should be a small chrome bullet-shaped cap on the vent. It's pointed at one end and open at the other. On most TriStars I've seen the open end faces down and therein lies the problem. (There's another opening in the hull close to it but that's the bilge pump outlet.) Sorry I don't have close up pics to illustrate.
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