Shasta man (name?)
You are correct about distance being a factor...but the equation is not simple. If an object is further from you, there is more water between you and the subject...so less color. However, if you are shooting up on a bright day, the subject will have more light so more color (and shooting down less light and less color). As a general practice, it is so much work (and weight) to engineer solutions to move the card away from you for white balance. Experience is a more effective measure.
What I do is consider where my subject is or will be (how far, how much light, shooting up, leve or down [try to avoid]). I then hold my slate and adjust the angle of the slate to capture more or less light (cooler or warmer). This will compensate for distance from the subject and whether they are well lit of not. If the slate isn't doing it and things are too red, I point my cam at the surface and balance of light at the surface. Interesting find: by zooming on the surface light (sun) you can cool the picture even more. This is very effective for very red coral environments and for shallow dives where mantas are flying over. Removes all that red burn out and pink water.
Again, having engineered some very complex contraptions for the slate, I can state unequivically that throwing all that way and learning and expirimenting with techniques has delivered far better results for me. A single method will only be good under certain circumstances. I use all the following for white balance depending on what I need to get it right: clouds, slate, water surface, zoom surface, boat bottom, my hand, sand, sponge coral, other diver, etc. Focusing on a single method will only make you good when that method is appropriate.