What you say is correct. There is an issue however that the custom white balance in a camera has usually a given operating range. Once you are outside of the color temperature range the adjustment is not as effective.
A filter has both a warming effect (reducing temperature) and tint effect. Once you are outside your camera temperature range the filter can bring this back in to operate custom white balance properly
So in general you can emulate a filter with temperature setting only on land not under water