Boiling water using electricity and then generating electricity with that steam makes no sense, since this process only has a very low efficiency (limited by physics at around 50%, in practical applications more like 20%). My question to you: If you already have electricity on the ship, why not use that to drive the motors directly instead of losing energy on the way by conversion?

There are a lot of ships that use a diesel generator to power electric motors (search for diesel-electric transmission). Some ships also use a steam turbine instead of the diesel motor in this configuration (turboelectric transmission): some nuclear aircraft carriers and submarines, and a few cruise ships (e.g. the RMS Queen Mary 2 - the combination of steam turbine and electric motors makes for a much smoother ride than a diesel engine)

Why don't all ships use this? Diesel engines (I would call them oil sludge engines, since that better describes ship fuel) use cheap fuel, are cheap to build, cheap and easy to maintain and the logistics to service and refuel them are set up. There is no real incentive to change this.

There are also some vessels out there using solar panels to generate energy, but at the moment, these are mostly smaller ships like ferries.