This guide will tell you where to find the Repulsion Cannon in Subnautica.
You will need to head about two bumps to the right of south or 2 in half bumps to the right of the south, starting from the coral tube sticking up out of the water. You have to keep moving until you reach a wreck in the grand reef.
It is recommended to have a sea moth with an MK1 depth module, a high-capacity O2 tank, a sea glide, and a survival knife.
The main threat you will encounter at the wreck in the grand reef is the warper, which can warp you out of your vehicle. If it does, you can either run from it or attack it using your knife, and then it will warp away.
Starting from the coral tube, you have to make your way about two bumps or 2 in half bumps to the right of northeast until you reach the lifepod 12 in the bulb zone.
The main threats you will encounter in the bulb zone are the ampeel; you have to avoid them and the bone shark you can either avoid or use perimeter defense against to get it away from you.