Whether it is Pot, Alcohol, or even some prescription drugs, impaired is impaired. The sad thing is that people who smoke every day fall into complacency. It is so much of their everyday lives that they don't realize that they are impaired (or won't admit it). Like the drunk who drives because he "only" drank a 6 pack.
When you are in a hostile environment, ie. can't breathe without life support (SCUBA), It is damn foolish to partake of any kind of mind/judgement altering substance.
It would be bad enough to have to rescue some fool who can't think straight. It would be even worse to have to count on them for help should you need it. Personally, I would find another "Buddy" that has their head on straight.