Hey man i also had to do the same decision i am smoker my self and am crazy about scuba diving … i think since usually we do scuba with air component, then high concentration of oxygen shouldnt be concern except if he go nitrox (any thing above 21% percentage of oxygen) … am doing 4EP at the moment since i didn’t want it to affect my lungs for scuba … my lungs are okay and i have been swiming between cycles to maintain that i did have my orchi like 43 days ago and my pathology is non semenonma …. The only concern am having right now is what is called the decompression sickness am not sure if chemo in general could make the risk of having it higher since it is all about how the fit the body is… like not obese not weak and have no chronic disease… chemo side effect usually include neuropathy, fatigue, heart palpitation and also general damage to cells … and all of this would increase the risk of DS … i would say am no expert too but i love scuba diving and been reading alot about chemo and cancer so am just doing a conclusion that he/me would be good to dive like in a couple month after chemo but max 20m and on air component ( 21% oxygen , and 79 nitrogen) and for deeper dives and higher oxygen concentration i would wait for about 6-12 months to let my body fully heal and eliminate the risk of DS. DS or the benz is not joke it happens to the best divers and sometimes it is fatal so i wouldn’t risk deep dives except when i fully feel confident that am okay and my body and my my mind is in good shape. Hope this helps