I know that Ranitidine is light sensitive, but my guess is it would have to be exposed for a good amount of time. I am not sure about water sensitivity.
If you are having feeding issues, I would recommend what I do if you don't mind filling your house with white dust (lol). I grind my horses daily doses into snack baggies using a electronic herb grinder: 10 pills (or whatever dose) + 1 lifesaver peppermint (these because they don't melt like the starlight peppermints). I then put all of the baggies into a larger bag and wrap it in foil. This is in the feed room at his barn. I also have a majorly picky eater. :)
ETA: If you're rolling in money, you can order the Ranitidine dose pre-ground with peppermint. But since I'm not, I do it manually.