Where the front or side yard is on a river, a private, in-ground swimming pool (including cabanas, aprons, decks, walks, etc.) or boathouses may be built in such front or side yard provided that in the R-30 and R-20 Districts, a side yard and front yard setback of 25 feet is maintained. In all other districts, side yard and front yard setbacks of 10 feet shall be maintained, however accessory structures such as cabanas and boathouses shall maintain a setback of 50 feet or the average setback of structures on the abutting riverfront property, whichever is greater. However, in no event shall the required setback be more than 75 feet.