regardless of the thread direction, water evacuation will occur since there are grooves on the tire's surface. under wet conditions, a unidirectional tire that used in reverse will have a slightly lower hydroplaning point (speed) compared to one that is used in the correct direction. unidirectional tires allow for higher traveling speeds under wet conditions but it doesn't mean that they won't hydroplane. they still do but at a much higher speed compared to that of ordinary thread designs.