You can't just throw in a bigger breaker, and expect it to work the way you want.
First of all, you're going to have to have the utility install conductors sized appropriately for 200 ampere service. Depending on how the building is wired, you'll also have to upgrade the conductors that run from the service drop to the meter (this might be handled by the utility, depending on how the building is wired). Next, you're going to have to upgrade the conductors that run between the meter and the disconnect (likely handled by installing the new panel pictured). Finally, you may have to upgrade some of your grounding and bonding conductors, as they are typically sized based on the size of the service.
Once all that's done, then you can likely proceed as you've described. However, you're going to have to evaluate how the system is grounded. The service neutral will have to be bonded in the new panel, where the 200 amp disconnect lives. Then you'll have to feed the existing panel (now a secondary panel), with a 4 wire feeder (hot, hot, neutral, ground). You'll also have to isolate the grounded (neutral) bar in the existing panel, if it's not already.
Long story short, this is not a simple job. There are a lot of subtle details, that can easily be missed by a DIYer. I would recommend having an Electrician at least help you plan the project, and inspect the completed work.
Your 200 amp service can supply your 150 amp water heater. But will anything else in your house require power at the same time? Such as a microwave, room or house heater, room or house air conditioner, stove or oven? Have you considered using the 24 instead of the 36?
Best Answer
You'll need utility help for this one
Since you're dealing with an "all in one" device that combines a meter base, service disconnect, and loadcenter, you'll need to have the utility cut power for you to be able to safely replace it. Fortunately, this shouldn't be an issue for them provided you have them do it during normal business hours.
Recommendation: don't go with another all-in-one
While all-in-one/combination service entrance devices are inexpensive and labor-saving for production builders, they have the great problem that you see here: you need to have the utility cut power just so you can expand this. A separate meter-main and panel gives you more flexibility in this regard, and also expands your ability to pick panels (I haven't yet seen a CSED upwards of 42 spaces, whereas many loadcenter lines go up to 54 or 60 spaces these days).