From your description of your training, it sounds like you need a solid multi-sport watch that is waterproof for swimming. Many watches are waterproof and can handle a swimming workout, but where they differ is the metrics that they can measure for swimming.
The Polar M200, which you expressed interest in, is a watch geared for running, but is waterproof up to 30 meters for swimming. It can track the time of your swimming workout, but doesn't track your strokes, which could be very useful information.
There are many other multi-sport watches on the market that can meet your needs. What you really need to think about is the information you want to know about your workout. If the length of your workout, and distance you go while running or swimming is all you need, a simple model watch is a good choice. If you want to know your calories burned, number of strokes, VO2 max, changes in elevation, or other more in-depth stats, you are going to want to seek a more advanced watch (which many times come with a higher price tag). Another item to consider is where you will be swimming- most watches will work in a pool, but not all work in open water.
Luckily, RunnerClick has done quite a bit of research on running and multisport watches, and the guides below may be of help to you:
You can also search for buying guides by brand, or even for reviews of individual watches.
Good luck with your training!