Signed up to go scuba diving with two others. I was certified years ago but had not gone diving in a long time. It was my girlfriends first time and her friends second time. Began training and felt very comfortable with the instructor. Eventually got out to the pool and went through all the hand signals ect. Before we knew it we began getting ready to jump on the boat. While gearing up on the beach one of the instructors knocked over an air tank which made me a little nervous because I know what can happen with compressed air. Shrugged it off and jumped on the boat. Upon reaching the dive site there was miscommunication between the instructors about tying up to a buoy. Our instructor was not pleased with the other instructor who seemed younger and less experienced. All gear was double checked and we were off. Our small group of three divers and one instructor were going to climb down the rope connected to the buoy for an easier descent. Upon entering the water I was the first to go down because I had some experience. I went down along with a cameraman who decided to come with us for photos. Very nice guy. After I reached the bottom I began to notice a stinging sensation in my hands. The rope from the buoy had sharp shells and barnacles that had cut my hands in three places without me even realizing. The exact same thing happened to my girlfriend on the way down as well. A small warning would have been nice as I am guessing this isn't the first diver it has happened to. Not to mention blood in the water usually attracts the wrong kind of attention. Upon reaching the bottom I found it very difficult to stay down even though my vest was completely deflated which I checked multiple times to see that all the air was out. I needed more weights to stay down. My friend was having the same problem as well. Once everyone was down we began trying to swim as a group which was difficult because 2 of us kept floating to the top. Within about 3-4 minutes the instructor checked my tank which was already down to a hair below 500lbs. Dive over for me. He brought everyone to the top and told me that I was going to have to go back to the boat because I used all my air. But before I went to the boat he asked for a weight off my weight belt for my friend. Obviously neither my friend or I had enough weights to stay down. An issue that should have been taken care of before we left. What was supposed to be a 40 minute dive for me turned into 10-15 minute fluke that actually felt like 5 minutes. I think there was not enough air in the tank for a 40 minute dive to begin with, another thing that should have been checked before departure. After getting back in the boat about 20-25 minutes went by before they surfaced. I really expected the instructor to speak up and maybe give me a partial refund considering the circumstances but no luck there. I will not be returning to Gili Divers in the future.