"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."

I'm not sure what your point is, but I want to make sure you realize that this doesn't in any way refute how a pipelauncher works. In fact this is unrelate to how a pipelauncher works. There is no doubt in my mind that a pipelauncher would work- for the purpose I've given it- namely to be a launch pad and to add some speed to a rocket.
A pipelauncher is mind numbingly simple- it makes car look extremely complicated and a very unlikely contraption in comparison. A pipelauncher is about as simple as simpliest rocket engine. A rocket engine was originally a doubtful concept, "How can you push against a vacuum", the supposed educated men of the day said. Doubting a pipelauncher could "work" is similar to thinking that steel boats can't float, sailboats will flip over, and that heated gas doesn't expand.

A pipelauncher will be made, as soon as someone who needs a launch pad, knows that a pipelauncher is an option. And then compares the costs of building a pipelauncher to building a static pad. If this person does not have "access" to lots of beach front land (meaning it's basically free) and wants to own (rather than rent) a launch pad, then a Pipelauncher is simply cheaper, faster, and better.
And since I know the Russian pay over 100 million per year to "rent" their crappy launch site. And I've heard of paying 4 million to launch a Delta II from KSC- renting is pretty expensive option even for a single launch- and we won't even go into all hassle those bureaucrats will dish out to the money grubbing capitalists, who dare to think they can go into space.