converting water to steam, especially very hot steam (like, 1000+) should produce a LOT more pressure than it currently does. for an example, look up my nuclear-thermal rocket. Its exhaust velocity should be considerably higher than one that uses a chemical explosive as its reaction mass, but it's actually somewhat lower.
I think, steam particles should increase their speed when heat is increased too (more heat - more speed), this will simulate expansion of steam and it will help your rocket
I know this thread is old but its a factor of momentum,pressure & gravity that more pressure should be created if water boils at a hotter temp and isn't released.