Instead of pressure, use a volume of what is already there, water. Put a submersible pump under the water and bring the cord out to what ever switch will be the on and off. Build one of those PVC rams out of say 2 inch pipe, all plastic and hook it up to the 2 inch line from the submersible. On equals quick movement off means gravity fall back into the water.
The creature might have a relief of even this low under 20 psi stream and puke water at the patrons once up with a simple open hole. The size pump is pretty much a budget thing and gallons per minute calculation. No high pressure, no long runs and anchoring of air lines. It only needs a big rock and a submersible pump. The big rock could be a poured concrete hunk that has the mounting for the structure of the body or the hinge it rises on or what ever.
Another fabulous post from the U.S.Department of Wild Imaginings, now in spectaclar stereo, sponsored by the Adhesives and Sealants Council, suggesting ways to stick things together since the 1800s. Not fabulous in a gay way. Your results may vary. Illinois residents add 8% sales tax. These posts have been made by professional post makers, do not try this type of posting on your own without extensive training, lovely assistants and a trusty clown horn.