Here's the checklist
One aft near the transmission & raw water filter for the raw seawater coolant. Make sure the raw water filter is clean before starting the engine, as seaweed & junk get sucked in (I had a baby fish once).One under the galley sink for sink drainage.One under the starboard settee cushion next to the holding tank & macerator pump.Two under the head sink. One is for the toilet, the other for sink drainage. Note the "Y" valve there -- the refrigerator shares drainage through that.Not a valve but still a through-hull: The speed/temperature sensor is under the sole hatch in the forward cabin ("V" berth). The sensor can be removed while the boat is in the water (the fitting has a little rubber flapper valve to minimize the gusher) and a dummy plug can be inserted. The sensor paddle wheel does collect seaweed & other marine crud (we get lots of brine shrimp!) & needs regular cleaning.In your C320 boat manual there are some fold-out diagrams of the plumbing where the positions of the valves & seacocks on the hull are illustrated. I used colored markers to identify & trace coolant, waste, potable, and "grey" water lines on the diagram to aid trouble-shooting.Each time to open/close the seacocks, just run your hand up from the valve & feel the hose for softening/cracking & the clamps for tightness. Good luck with your new C320. It's a great boat.--RonC320 #831, "Lady Jane"