The town of Ardrossan hosts two jetties, a large one with a max depth at 15 m/49 ft and a small one with a max depth at 6 m/19 ft. The large one to the south is the grain jetty, the other provides local fishermen with a jetty fishing spot.
Both of these are worth diving, especially the end of the long one is really pretty. Use a boat to get there or snorkel over to the jetty and approximately halfway before dropping down.
Obviously this jetty cannot be dived when there is a ship in port. The smaller one is home to a lot of life as well (especially on the ground), and makes a real easy dive, due to almost non-existing currents and shallow waters.
The spot is suitable for practitioners of all levels of difficulty, however visitors need to bear in mind that sometimes basking sharks come into Ardrossan harbor to feed on plankton.