Problem solved! Thanks all for your thoughts and ideas on dealing with the air leak somewhere around my Yanmar engine that prevented the proper flow of raw water. With the advice and help of Steve Davis ("Aloha"), I went first to the most likely place ("Are you sure the O ring is properly seated in the impeller housing?" asked Gary Silver). When I last changed the impeller in the middle of the North Atlantic last November, I neglected to also check and change out the O ring. When I pulled the cover of the housing, it was fairly corroded so I cleaned it up, lightly sanded the surface, and replaced with a new O ring.
Further, while reading the instructions for the use of the "Speed Seal" cover (installed by the previous owner), I saw that I was supposed to have a white plastic washer underneath a metal washer--both--not one or the other. The plastic washer was missing and found later on the floor underneath the engine. Also the inside of the Speed Seal cover was pretty worn. Thus, I decided to replace the speed seal with the original cover (happily left among spare parts by the previous owner).
Though the impeller was in good shape, I went ahead and changed it out and then put everything back together. During this process, of course, all the water had drained from all the hoses and the pump. So the real test (to see if the impeller housing and cover was the source of the air leak) was to start the engine and see if it self-primed--and it did! Works like new now.
All that said there were several thoughts on other potential sources of the air leak and while, so far, it seems to be the impeller housing, I will next check all those areas to ensure they are and remain healthy (e.g., the boot area mentioned by Bill Rouse and other areas where I'm likely to spring a leak).
I posted some photos here:
Rick and Linda Grimes
"Rascal" SM2K (#404)
Sun Bay Marina, Fajardo, PR