Max Velocity of a Water Rocket

[flash]

Find the maximum velocity of a PET bottle rocket on a day with temperature 30 degrees celsius and pressure 1 atmosphere, if the pressure inside the bottle was 40 psi and the volume of water in the bottle was 300 mL.

I know of Tsiolkovsky's equation but I'm not quite sure how to use it here. Any help appreciated

EDIT: Pretty sure I need to find the exhaust velocity of the water given the above details. I have a formula that says $$\Delta p = \frac{1}{2} \rho v^2$$ where $$\rho$$ is the density of water and p is pressure that (I think) gives the exhaust velocity v but I don't know where the formula came from or if it is even correct.

 

[flash]

Feel free to ignore the first part of the question. I just need someone to explain that relationship between pressure and exhaust velocity there. Also I'm guessing that the density of water changes with temperature?