I suspect you'll be faster than that in the water, I can barely swim and did my first 750m OW in 16 minutes. The wetsuit helps, and I am definitely faster in OW than in the pool. All the guys I know who have a swimming background leave me for dead and are sub 12 minutes.
The one place where you will lose time is in transition. This varies from tri to tri, but includes the run from the lake to the transition zone, which in some cases can be a considerable distance. Add the fact that it will be your first time, so you're bound to fluff something or other (don't ask me how I know this

). Plus as FM points out, there's the transition effect of getting your body to switch from different types of activities. Dont' be surprised if takes you a km or so to get into your running stride.