It might look confusing because the weight of the piston is only about 10 gr. But at speed of 35 m/s it would have 6,1 J of kinetic energy. It is same like if you let some weight of 1,2 kg to fall down from 0,5 m height.
Puede parecer confuso porque el peso del pistón es de solo unos 10 gr. Pero a una velocidad de 35 m/s tendría 6,1 J de energía cinética. Es lo mismo que dejar caer un peso de 1,2 kg desde una altura de 0,5 m.
If somebody want to make a calculation here is a formula, it is really simple:
p1 = p2 + 1/2 q vi^2 - 1/2 q v^2 = 1 bar + 1/2 q (vi^2 - v^2)
p1 - pressure of water in muzzle
p2 - outside pressure, it is small, can be omitted in calculation
vi - the speed of water leaving the muzzle
v - speed of the piston/water being pushed by the piston
q - (1020 - 1030) kg/m3 (for sea water)
vi = A/Ai*v
A - cross section of the barrel (piston)
Ai - cross section of gap where water can exit the muzzle forward