Are 2.5 Gallon Water Jugs legal? We are planning to use the bottom half as a container to hold balls on our robot.
Assuming you’re building a robot for a 2014 off-season event then I see no reason as to why a 2.5 gallon water jug wouldn’t be legal. Just make sure it fits completely inside the frame perimeter and isn’t sharp and you should be fine.
Edit: I didn’t read your user name before commenting 
Sorry about that!
Based upon Game Manual Part 1, <R04><b> it would appear that yes, this is legal.
You should Q&A it though, as I haven’t done FTC since 2010.
Since you appear to be asking about FTC material utilization, I looked in the FTC manual. Keeping in mind that I’ve never competed in FTC, it looks like this would be a COTS post-processed material with 0 degrees of freedom (since it has no joints or hinges inherently), which would be allowed under R04 parts B and C, if it was obtained from a standard distributor and would be considered obtainable by the majority of teams.
I am guessing that the applicable rule is R04
b. All post-processed materials are allowed provided they are readily available to the majority of teams from standard distributers (e.g. McMaster-Carr, Home Depot, AndyMark, etc.).
Examples of allowable post processed materials are:
perforated sheet and diamond plate
Injection molded parts
3D printed parts
I am guessing the part in question is an injection molded part.
Just don’t put water in it… Or do! (Surprise watergame!)