if the velocity of river is 6i^ m/s and the velocity of boy in still wateris (8i^ + 6j^)m/s. then the velocity of boy w.r.t. river water is.should it be same (8i^ + 6j^)m/s?

Yes.

Because the swimmer is trying to swim with the same speed as he swims in the still water. It doesn’t matter whether the river is flowing or not, the velocity of the swimmer with respect to river will be same. However what the observer standing on the bank will be seeing a different velocity which we call actual velocity of swimmer as it is the velocity with respect to ground.