I have to do a John Cormack on you, gbaikie.
Sorry.

Assuming deep sea-water is just above freezing [274K],
and the GMM of the water molecule is 18.0gm/mole,
the "rule-of-thumb" equation for explosive velocity of the molecules [km/sec] in a vacuum at that given
temperature is c = 0.25[274/18]^1/2.
c = 0.25[15.2]^1/2 = 0.25 [3.8] = 0.95Km/sec.
c = 950m/s

Water-molecules explosively propelled by extreme pressure differences at temperatures just above freezing cannot exceed 1,000 meters/second [2237mph].
The Specific-Impulse works out to (950m/s)/ 9.8m/s^2 =<
100-seconds of impulse, thereabouts.

Sorry to splash cold-water on your concept, gbaikie.

Jacob