NET
6 But he must ask in faith without doubting, for the one who doubts is like a wave of the sea, blown and tossed around by the wind.
Darby
6 but let him ask in faith, nothing doubting. For he that doubts is like a wave of the sea driven by the wind and tossed about;
KJV
6 But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed.