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.

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.