Well, I can't really recommend any particular pain medication, but there are several that would work. As for water on the knee, it's just fluid that collects around and under the kneecap. To have it removed, (***warning for the squeemish, this ain't pretty***) the doc will insert a fairly large needle just under the kneecap, with the plunger on the needle already depressed. Once it's in place, the doc will pull the plunger out, essentially drawing that fluid into the syringe. Quite frankly, it hurts like hades, but only while it's going on. The next day your DH will feel better than he probably has in ages.
I don't know exactly what causes this inflammation, but the times I had to have it removed were all sports related (back in my HS days of playing football). I guess it's a pretty common thing for pro athletes, particularly football and basketball players, as I know several players that have to have the procedure done on a fairly regular basis (Brett Favre had to have it done every week a couple years ago).
Hope any of this helps.
PS....A knee brace is a must for your WDW trip....I never tried a patella strap, but just a cheap little Ace knee brace will do wonders.