It's hard to compare pool times to open water because there are a lot of differences in the environment that change (such as wind, waves, current, temperature, etc). For the pointy end swimmers (The ones that will be first out of the water), there isn't that much of a dropoff in the actual swimming time. The difference in a pool swim is mostly due to being able to push off the walls at the 25/50 mark.
The less experienced you are, the greater the difference will be. Since you have ~ a 15 min swim time for an 800 (Which is a decent time), I would guesstimate you will be able to do the swim in a lake environment in about 18-20 minutes. The variance will be on how well you swim without lane lines, how well you sight, and if you get nervous/tense in the open water (As well as weather conditions race day such as wind/waves).
The major differences will be:
- Temperature: Most of the time the lake/ocean will be much colder than a typical pool. If you swim in a competitive temperature pool then the difference will be much less. Most commercial pools are 80+ for degrees, competitive down around 75 degrees, and lakes usually run between 65-80 depending on depth, time of year and geographic location. (For example, the shallow, man made lake near me in Arizona that hosts a lot of triathlons is not wet suit legal for most summer events, as it is too hot).
- Visibility - You will rarely ever have as much visibility in a lake/ocean as you do a pool. Most of the time it will be in the range of a few feet.
- Waves - Even a lake swim will have wavelets and/or waves, especially if there is a wind. These will tend to push you off course, can interfere with breathing, and require a lot more effort if they are of any size.
- Sighting - You will need to do a modified stroke every so often to make sure that you are still swimming in a straight line. Very few swimmers can swim straight without guides. One way to check your tendency is to swim in a lane with your eyes closed. Count how many strokes before you hit a lane line. That's how many strokes you want to take between picking your head up to look. The better you are at this technique, the less time you will lose.
As far as drafting, it's very effective. You want to be more to the side than directly behind, because of the bow wave effect, but even following toes can help. However, if you find yourself constantly tapping their feet, you need to get out of the draft and find faster feet. Even in a pool, if you watch high level competitions, competitors will hug the lane line to get as much draft effect as they can if they are behind.
I would recommend getting out and doing open water swims as practice, and seriously consider a wetsuit. A wetsuit is like legal cheating for swimming. However, don't do it for the first time on a race day. A swimming wetsuit (Don't borrow a surfing one) fits very tight, and can produce hyperventilation and claustrophobia if you're not used to it). As far as workouts, practice doing a short 100m sprint, and then the rest of the distance straight, with occasional sprint surges of 10-15m every so often. This will simulate your race environment, with the start, surging to find feet or drop people, etc.