Triathletes (who do both of these exercises) typically swim first, and you will notice it's always first in races. There are a few reasons why I support this:
-- Cycling is easier on the body than swimming is. You do the harder exercise (swimming) first, when you have energy for it. That way you can get more out of your swim and overall get a better workout.
-- If you are doing open-water swimming, the risk of drowning increases the more tired you are. So you want to swim when you are less tired.
-- Swimming lowers your body temperature because of the water, whereas cycling (and other non-water sports) raises your temperature. When your body temperature is low, it makes you feel artificially hungry. Heat sports on the other hand make you feel full. So if you swim AFTER cycling, you will end up feeling really really really hungry. (According to articles I've read AND personal experience. I always feel staaaaarving after I swim in a cold pool.)