Do you know a simple fast front-end MD software for OS X
I'm biased, but I recommend Avogadro which can do simple force field optimization using MMFF94 or GAFF. It's not MD, but it would certainly work. It runs on Mac.

The water molecules shouldn't "fly away" during minimization, since there should be hydrogen-bond and Van der Waals terms attracting them to each other and to the protein.

I'll also point out that VMD runs on Mac and are quite easy to use. It can set up MD calculations with NAMD, GROMACS, etc.