Here are some thoughts on dueling. Sometimes there are down times with only one or two other people available for a water fight. Such a small number of people make a good water war impossible, but a good water duel becomes an option.
When it comes to dueling, weaponry must be balanced. There is little point in practicing one's water weapon skills against a poorly armed opponent. Also, unless one just really wants to drench each other silly, the weapons used for dueling should not be too large. Personally, I would recommend something between an XP 110 and a CPS 1000. Anything smaller than an XP 110 does not have enough power or range for a good duel. Anything larger than an CPS 1000 will drench single or double opponents too quickly and make staying dry nearly impossible.
My personal favourite weapon for duels is the SC 500. The water capacity is not very high, but that means one has to take more care when firing. That way, one must improve one's aim in order to be able to properly soak any threat. When one is good with a SC 500, one can unquestionably drench an opponent using a higher class weapon such as a CPS 1500. The other nice thing about the SC 500 is, of course, its quick-charge ability making dueling much more continuous instead of having to wait too long when refilling.