I have actually never done anything but freedive. If u gave me a tank tomorrow and told me to breath off it and dive, i would probably end up accidentally killing myself.... I'll be totally honest, 20 people on only a 65 foot boat sounds pretty uncivilized. It will either be a lot of fun, or a complete shit show. I would say it's about 16 people too many.... All jokes aside, stay safe, sounds like it could be a little sketchy as well. At the very least, don't let them forget and leave u out at sea....