This video shows the sort of obstacle you can (literally) bump into when bottle diving - a sailing boat moored to a sinker by means of a riser chain. Although it was obvious from the surface, once underwater it is difficult to correlate its position and it is most practical to mark it by survey.
In this clip I start at the bottom and move up the chain to the buoy at the bows; then I circumnavigate the vessel from the port side, moving round the stern, and return to the bows by starboard. The keel is clearly visible and the rudder and propellor can be seen at the stern. I finish by going down the riser to where I started.