Hello...
for a given runway length ...and a given aircraft weight....on a wet runway condition...
actually ...the wet runway defenition is :a runway is conidered as wet when the surface has a shiny appearance due to a thin layer of water.when this layer does not exceed 3mm,there is no substantial risk of hydroplaning.
wet runway takeoff is always associated with a lower V1 than a dry runway. the reason that if you reject(abort) the takeoff on a wet runway using the normal dry runway V1...there is a big chance that you might over run the runway...so for this reason the V1 has to be reduced with respect to the Vmcg.
and to maximize the stop margin in case of an aborted takeoff ....a higher thrust is required to reach the V1 earlier...so that if you abort...you will have enough stop margin .
Good Luck and Best wishes.