Today's Air Command site update has some great photos of the Polaron IV clustered and staged water rocket. They use different colors in the boosters so each water jet can be distinctly seen at launch. They also have nice shots of the staging. Design-wise, they have plans for home made 'tornado' couplings. Theirs are made from hardware store components and I assume equivalents can be found here in the States. I embedded their diagram to the right.
BTW, 'tornado' couplings are so named because if you join 2 bottles, one full and one empty, and turn the assembly over like an hourglass, then a tornado forms in the top bottle as it drains.
Oh, if you want to see their updates directly, you can subscribe to their blog feed. This doesn't have as much info as their main site, but does give a quick-look at their progress.