Can you try it in some fresh water somewhere?

If it's not designed for a triathlon, it may be a little loose, but odds are that it probably won't make a huge difference (unless you're an elite swimmer who'll notice tiny differences). I've experimented with various tank tops, and while they felt a little funny, I don't think it made a significant impact (but keep in mind I'm a weak swimmer and am not going that fast in the first place). Another option is to swim in a sports bra, then throw on a top quickly before you get on the bike.