Xamarin - Remove SearchBar Underline in Android

You're right about creating a custom renderer, but in your renderer, to remove the underline, we need to find the plate of SearchView in native android first. You can for example code like this:

protected override void OnElementChanged(ElementChangedEventArgs<SearchBar> e)
{
    base.OnElementChanged(e);

    if (Control != null)
    {
        var plateId = Resources.GetIdentifier("android:id/search_plate", null, null);
        var plate = Control.FindViewById(plateId);
        plate.SetBackgroundColor(Android.Graphics.Color.Transparent);
    }
}