Yeah, when I first read about the diesel-electric concept (actually in a book about railroad locomotives) it made no sense, you're converting mechanical motion into electricity then converting electricity back into mechanical motion?!
Oddly there isn't as much loss of energy as you would think, it's a LOT more reliable and durable than all those u-joints and clutches, and cuts way down on maintenance. Why they attempted the dual drive abomination on the first SARGO boats is a mystery, to me it should have been obvious after the success of the PORPOISE and SALMON class boats.