If a Pokemon used soak on my grass type Pokemon,would fire type moves be not very effective?

Question

Every time I battle a wild golduck in Pokemon black 2 it uses soak on my serperior.But when I click Pokemon,serperior,summery it says he is a grass type.

Answer 1

Okay, the basic mechanics of Soak turns your Pokemon's type into water for the remainder of the battle (as long as it does not switch out). Once the battle has ended, Soak's effect will have ended.

When Soak is in effect, everything the Pokemon does will be the same as a Water-type Pokemon in terms of battle mechanics. It will not gain STAB from anything else apart from Water-type moves, it will have a resistance to water, fire, steel and ice, and it will be weak to grass and electric.

Everything is calculated as if it were a water Pokemon.

But, as you say, when you check the summary, Serperior is a grass type because Soak wears off after a battle.

Bulbapedia on Soak

Answer 2

Yeah, you become a Water-type Pokemon, so Fire-type moves are not very effective.

Source: Experience