The water only adds a slight amount of steam to the burning air-fuel mixture and doesn't reduce the engine power appreciably. Furthermore, the engine is designed to extract excessive water, snow, or hail from the air volume before it reaches the combustion chamber. Bypass flow never hits the combustor since it goes around the core engine. This design provides the combustor a sufficient safety margin, says Wadia, that it can function under worst-case conditions. In summary, jet engines evaporate small amounts of water.
Engine design extracts large quantities from the compressed air before burning the air-fuel mixture.
That's how jets continue to function with snow, hail, or rain falling all around them.