Water filling on the launch pad! 

And here it is:

The main key to it is the use of a T joint (on its side) to build the water/air separation system. The launch tube is a irrigation system vertical riser. It is glued (using epoxy glue) into the lower part of the T joint, making a complete seal around the launch tube. Air from below enters the launch tube only. Water entering the side tube only goes up (outside the launch tube). Hence there are separate water and air paths into the rocket on the outside and inside of the launch tube respectively. Another T joint below divides the air between the air inlet hose and the pressure gauge.

A pressurised weed-sprayer is used to push the water into the rocket. A spray nozzle is modified as the water feed. An irrigation system tap is used to seal off the water feed when the rocket is pressurised. 

The launch system is the standard hose fittings. A brass fitting is used (I found that plastic fittings did not last). The fitting is the type with plastic retaining clips in it - I have had problems in the past with the fittings that used ball-bearings as they were prone to jam. Cable tied and glue (hot-melt) secures some cord to the hose clip which is then attached to the release tube (for want of a better name). This tube is mainly used to provide a clean path for the rocket's trailing ring fin, so that it doesn't catch on the hose fitting during launch. Bungy cords normally pull the release tube down allowing the hose fitting to open. A hinged lever under the release tube prevents this happening. The launch string has a small plastic strip at the end which goes between the hinged lever and release tube. When the string is pulled the lever opens, the string pulls away (hence there is no way launching will pull the launcher over), the release tube is pulled down by the bungies, opening the hose fitting, and away goes the rocket!

Hoses used are simply normal reinforced garden hose. The air supply is through a car tire valve stem (ask nicely at the tire shop), trimmed down and glued and hose clamped into some hose. Garden hose goes through all the launch tubes etc to the hose fitting where it is glued (as well as the normal retainer) in place for extra strength. The launch tube (which runs right from the base) is two irrigation risers - these can be easily joined together. The vertical tube (black) is a larger diameter irrigation riser pipe. Caps are screwed to the ends - an end-cap at the top with the hose fitted glued (hot-melt) to it. A screw on hose fitting at the bottom secured in the base (wood and hot-melt).

This diagram shows how the launcher quick-fill system works: