you don't start with floaters
You start with self replicating molecules that reproduce rapidly enough that you don't miss them if a few thousand, trillion, trillion... of them get fried.
Likewise, lots of molecules don't mind a bit if you raise the pressure a few hundred psi. Same with temperature.
Likewise, maybe there are metastable regions where a microorg could count on years of relative peace, at the right temperature and pressure.
After a modest start, the rest is just strategy.