Hi, I have seen multiple explanations as to why there has to be a maximum length for functional snorkels. 1. You inhale the same air that you exhale. (I.e. the so called "dead space" is too large). 2. The extra pressure, provided by the depth, on your chest is too overwhelming for your muscles to perform work against. I'm sure both are factors, but which would dominate? I made a quick estimate for 2. which maximizes realistic length at about 0.5m (all below the surface). About 1. Wouldn't partial pressures equalize quickly enough through diffusion? Or is the gradient too small? 1. and 2. are possibly coupled. Because if diffusion would be enough, number 2 would be somewhat redundant -unless- expansion through muscle work is required to increase diffusion area or decrease diffusion length or alter pressure (if it plays a role). If passive diffusion is not enough, convection would presumably be the main purpose of muscle work? #1 could possibly be overcome through breathing out through the nose while breathing in through the mouth.