I don't think the mulch will guarantee that it won't sink - I wasn't considering that. I rather doubt that it will, but if you have a very high subsurface water level, things get unpredictable. Having a raised mulch bed under the pool should make it easier to level the pool, and to keep the water from puddling in a single spot. But there's no way to prevent some of the consequences of adding water to ground that's already soggy.

The good news is that, compared to other pools, sinking is far less of an issue: if there's a problem, you can fairly easily take the pool down, correct it, and then put it back up. Setting up an 18' Intex, once you've done it, is probably not more than a 2 hour job for 2 people.