Unless the vessel has a high wattage requirement for other things, besides propulsion, then it's not justifiable to consider a reciprocating/electric propulsion setup. Many ships do use diesel/electric arrangements, but typically these vessels also have a huge need for electricity. A cruise ship, for example, may use over 50% of it produced electricity on the ships needs, rather then propulsion, making the setup viable.
It's all about the loses incurred with power transfer. The simplest form is a reciprocating engine with a shaft attached to the crank and a prop on the end. There's some frictional lose at the cutlass to hold the shaft and prop slip, but that's it. This setup is on used on small engines, in small boats and offers no reverse, gear reduction or neutral either. The next step up would be a transmission, between the shaft and the engine. More frictional loses in the transmission, but you gain torque multiplication, neutral and reverse too. Now couple the same engine to a generator. More frictional loses with the generator, possably the need for a transmission (read more loses) or PTO, so you spin the generator at the proper speed and a motor with a prop attached (more loses again). The net result is less then a direct or nearly direct coupling. There are other loses too, but you should get the idea, which is why, unless you have a substantial need for electricity generation, other then propulsion, such as huge refrigerated fish holds, or massive pumps or hoisting gear, the trade off isn't worth it for just propulsion and running a toaster in the galley.
