I totally understand how they can be frustrating. Try drawing it out as a picture. Draw a straight horizontal line to show where surface of the pool is at. This line is considered to be at 0 meters. Since the swimmer is measured to be 2.3 meters tall with their arms stretched up, and the diving board they are on is 10 meters high, their maximum height is 10m+2.3m which is 12.3m. The depth of the pool, since the surface of the pool is 0m, would be -5.5m (don't forget to always include the unit of measurement, whether it's feet, meters, inches!). Since the diver goes from 12.3m to -5.5m, I would find how far they traveled by writing -5.5m-12.3m=-17.8m; that is, the driver traveled down 17.8 meters.
