Except that there's an important theological difference between the mikvah and baptism. Baptism isn't a purification ritual. It's a sacrament that points to grace (purification -- although not as the Jews would have conceived it) that has already been effected. There is no power in the act, in and of itself -- only as it points to what God has already accomplished.
I agree, which is why I stipulated that, while the ancient practices might have been similar, I didn't see much commonality between the more modern practices.
I probably should have been more specific in my post.