What is the best way to train in a pool for an open water?

Here's what I'd do. No guarantees on it being "best". 10 x 100 on whatever pace gives you about 5 seconds of rest. If you start getting more than 10 seconds rest on your last few intervals, drop the interval by 5 seconds. Learn to pace. You will be faster with a consistent sustainable effort rather than going out too fast at first and suffering at the end (or going out too slow at first and having a lot of energy at the end). Pools with pace clocks are great for doing this. Try to get all your 100s to be within a couple seconds of each other. Take a break and then do another set of 10 x 100... or do some kicks. 10 x 100 gets boring kind of fast. Get in a timed 800 once or twice a week. Take an open turn at each 200 to get your splits. Or even better have someone get them for you. Make sure your pace is keeping steady throughout the 800. Sight an object at each end of your lane at least once per length. You don't want to learn how to sight on the day of the race.

Great suggestions so far. And you know, the one about the cannonballs isn't far off. See when a kids' team is practicing and hug the lane line in the lane closest to them if you can. They can still create a lot of churn even with the lines. Another good set to do would be to do an 800 by doing a very fast (90%) 100, then settle into a good pace for 500, then start to pick up the pace at the 600 point (85-905), then sprint the last 100 with all you have left. It'll simulate the start and end, then a solid pace in the middle.

the one about the cannonballs isn't far off. and disturb a hornets nest somewhere on deck, get stung a few times then jump in:)

Maybe take out the lane lines and get some kids to toss some cannonballs in from the side to simulate waves? :)

For a ½ mile open water swim, I suggest training as you would train for a pool mile. Also incorporate sighting into your drill sets. Many people recommend alligator sighting (periodically raise your head a little so your eyes are looking in front of you; try to keep nose and mouth underwater). I prefer shark sighting (after a breath, rotate your head forward to sight then lower your head as your arm finishes its recovery).

I would swim sets of longer distances of 2-3 times the distance you are going to swim. I think picking your head up higher to see the markers on the water would keep you from sucking in too much water. If it is dead calm ,a lower head position is best, but open water will have waves

What about sets where you "turn" a few strokes before the wall (no walls)?

I've participated in three open water swims in the last year or so and I trained for all of them in a 25m pool for the most part (I went to a 50m pool a couple of times). My guess is that you're not good at sighting yet. You should be able to find some videos on YouTube. Actually, I've embedded a good video below. If I'm right about your sighting, you might want to find out about the course and think about strategy. For example, when I swam the swimming portion as part of relay team in a full distance triathlon, I talked to people who entered the same race the previous year. I found out that I could follow a wall of the man-made lake if I went to the far right on the way out, and then the wall on the opposite side on the way back. I breathe to the right so I couldn't follow the rope that is to the left of swimmers (in the middle of the course). Starting on the far right at the beginning turned out to be a great help in that I avoided the fight at the beginning of the race. Once you get started and figure out your sighting, your swim shouldn't be all that different from what you experience in the pool. Just find your rhythm and go! Good luck. www.youtube.com/watch

I only just got into open water swimming this year, so I don't have a ton of experience to draw on--but I do vividly remember what the biggest adjustments were for me since they weren't so long ago. The swimming is a hair different with chop and all, but nothing my swimmer's instincts couldn't handle. The really big difference, for me, was adjusting to very different visual cues. In the pool, you're following a black line on the bottom, which you see through very clear water. In the open water, visibility underwater is often quite limited and you've got to catch glimpses of your path when your eyes are out of the water. So I followed the sighting advice in the video that's already been posted here, and I also practiced swimming with my eyes closed. The drill I saw suggested was to do entire lengths eyes shut, and I did some of those, but what I think was most helpful was to combine that with a sighting drill: whenever my head was submerged, I closed my eyes. I opened them for every breath, and I practiced sighting every other breath. Then, as I got more comfortable with sighting and keeping myself on a straight path using only the visual cues that I got from sighting, I pulled back to do it every third breath, or every fourth--still with my eyes closed whenever my face was submerged.

You are more than welcome to come over to the MLK Jr pool in Long Beach, CA. It has no lane lines and no gutters. I highly recommed this pool for training for open water swimming. Plus its open year round. Indoor, showers lockers etc.