Generally, keep equipment higher up and sheltered from the elements.
The comment above about being unsympathetic to the desire for a foot-switch is not far off the mark too. A foot-switch is a comfort, and (as you have discovered) Field Day is not about comforts. If it's dry and pleasant out then sure, use a foot-switch. If it starts getting damp, unplug it and live with the reality that life is sometimes cold and wet.
When it rains heavily, keep electrical equipment dry at all costs. Keep batteries higher up (on a desk) and protected from water, while allowing some of that horrible cold air to circulate around them (lead-acid batteries can give off explosive hydrogen when under heavy current loads). Be especially careful if you are using a generator - it might be weatherproof, but the 110v it generates is going to cause some problems if it starts finding itself submerged. If you do use a generator, make sure it is weatherproof, and keep it on a table outside. Run the 110v lines high up, and drop them down to the equipment that needs it. There are reasons why the electrical utility companies tend to take this approach. All outlets inside should be high up and kept dry.
Keep yourself and the team warm and dry - there is no point in being miserable through the exercise. And keep your spirits up! If you follow these guidelines, you can enjoy the experience, and will probably be doing a lot better than the other teams in similar conditions.
Always try to enjoy the predicament, and remember that you are doing this out of choice, for fun. It will help you get more from the whole event.