One would suggest he can be best described with a word beginning with a P and ending with a K.
Not really, a great many people are totally mystified by electricity, just look at the perpetual question that crops up after advice is requested, and someone mentions "BEC". The fact that he did what he did indicates that he received incomplete advice, or had not remembered enough of it. I expect that after a little instruction from Dave, he will have the proper way lodged firmly in his mind now.
He probably made a better looking boat than I normally manage, anyway.
One battery for simplicity, provided it can handle the current. Two for reliability. If the two can be arranged to feed the system BEC via diodes, so much the better. Assuming that the one is the same capacity as the two added together, the two arrangement gives better flexibility for siting and weight distribution.