@Ward H @ Richard19068 and @ Scott T-Bird I had already left for my boat before these posts came in, and as a rookie, I honestly didn't look closely at the 12 volt meter to see if it read above 13, I thought anything above 12 was fine. I will be back at the marina on Sunday and will look then. In the meantime, the new Interstate dc 27 is in place. The diesel fired right up, I sailed for 3 hours, and then as I approached the harbor, the diesel fired right up with no hesitation. No "wawawa" during the restart. Of course, one ride does not make for a reliable test, but I will advise the forum.
Right now my switch is set to 1 + 2, The new battery is on 2. I will try the "2" and "1" positions singly to see if the old battery seems weak. I know the prudent thing is to remove the old battery and have it checked, but I'm not sure I am ready for the bilge. I am 63 and my body is starting to remind me of that old Bob Villa show "This Old House". When the soreness subsides, I will be ready for round 2. I am thinking that replacing the two year old battery with another new Interstate dc27 may be a good use of $100. Right now, and it may be soon to tell as this is not a measure, but my house lights are brighter with the switch set to "2" and the new battery, then when set to "1" with the old. That leads me to suspect the old battery is weak.
About the 12 volt meter reading 12.7, the existing stereo was playing when I took the photo of my electrical panel. It is an old car stereo that is wired to the 12 volt system. I don't know if this would cause the reading to be low.
Thanks to everybody for the assistance!