.....and learn how to use it. It will make your diving more enjoyable in many ways. It will extend your bottom time. It will free you from reliance on dive tables. It will help you monitor your ascent rate. It will allow you to go back over your dive profiles and see what you may be doing wrong.
Every peice of gear can be rented, including computers. But you must understand a computer in order to use it effectively. They don't all work alike. Buy your own and familiarize yourself with it. The kind of mistakes you can make during a dive due to an unfamiliar computer can end up getting you bent or worse.
For example, I recently dove on a liveaboard with someone (not my buddy, just a guy on the boat) who did not recognize that he had gone into deco on his unfamiliar computer. Not only did he go into deco, he failed to do the required stops. He ended up locked out of his computer and prohibited from diving by the boat's captain.
Buy your own computer and learn to use it.