Water can and will change the way fuel is used by the engine but you would have noticed it from the moment you started the plane (that is for sure)- And unless your fuel looked cloudy it was probably not that contaminated. Water and methanol are completely miscible (meaning that they stay in solution when mixed) But water does not mix with the castor oil which causes it to look milky. Your engine would not and should not quit unless it was loading up or out of tune.
So I would look for another cause before blaming others.
((IF)) you believe it is the hobby shop's fault (NEVER YOUR MISTAKE of course) than everyone who buys fuel WILL be having this issue just like you. Is that happening? or is it just your bottle? If that is occuyring than they owe you some fuel if not its time for an engine checkup.
How to store glow fuel
http://www.geistware.com/rcmodeling/.../fuelfact5.htm
http://www.rcuniverse.com/forum/Rc_F...m_40640/tm.htm
http://www.uoguelph.ca/~antoon/hobby/hhdfuel.htm