Hi Boris,
I carry 3 fire extinguishers at this time on Sea Angel (CD25). Similar to THATAWAY, my first is placed to the port side of the door as pictured in my pix album, page 2 of 3, pix #34, centered by the window frame, and with the supplied hardware, screwed into the bulkhead between the window and door frames..
The second is on the head bulkhead, outside forward, toward the keel line, above Wallas/Galley cabinet, centered with the inside head window.
The third is presently not mounted, but plan to place it near the helm, outside of the V-berth, on one of the bulkheads. All extinguishers are 2# units.
These three positions should give me immediate access to cover a galley or electrical fire inside and out. I choose to have my extinguishers mounted upright for two primary reasons. First is the speed of having my hands and extinguisher in the proper position when I grab it. Second is the possibility of having some of the soda/agent clog the discharge port if stored horizontally. And, yes do shake them several times a year - I try to do this at time changes, around July 4th and News Years. By setting a schedule like this I am sure to get it right at least a couple times a year; like changing the smoke alarm batteries. I also note this in my log book and 'things-to-do' note book.