The typing of the Pokémon changes to pure Water-type
Tested it through a replay also. As seen in the replay, Pelliper used Soak on the Slowbro, and its typing changed to pure Water-type.
Alsom if you check Bulbapedia's Soak page:
Soak changes the target's types to pure Water-type.
Therefore, the typing would change to pure Water-type always.
Hope it helped.
