math.pow() will not work with rasters, only with numbers.
However, the Power tool can take value or raster inputs.
This used to be the pow() function in GRID and Arc 9.x map algebra. I miss that.
and, Python string substitution is your friend. compare these two:
hydRad1 + '* ((' + flowAcc + ' * ' + cellSqMilesStr + ') ^ ' + hydRad + ')'
"{} * (({} * {}) ** {}".format(hydRad1, flowAcc, cellSqMilesStr, hydRad)