Mart,
I am suggesting reuse of the water for the main engine. A lightweight condenser would be needed, perhaps the rotors or other structure could serve as a condenser with a minor weight penalty.
A very light FLASH BOILER would be needed. (read about at your link)
I believe the thermal efficiency of a modern steam turbine is much better than a gas turbine. The savings in fuel weight could offset some of the system weight.
Also, I think the change of state from water to superheated steam is an advantage over hot air gas turbines. Check your statements about latent heat.
It's been 30 years since I studied marine steam turbines.