You also have to factor in that in open water swimming (assuming you are swimming freestyle) most of your work is coming from your arms. In a pool, turns make use of your legs a lot more than your arms, so your arms get a break. All other things being equal, I would say the pool is definitely faster just because of less arm fatigue from the turns.

That said, I'm slow no matter what I'm swimming in, so take that for what it's worth...