Waterproof lock for gate

My house is on a steep slope and my back gate leads out onto washland. It's very attractive scenery and I often go for walks out there. However 2-3 times a year it floods right over the top of the gate. I have a padlock on the gate which seems to tolerate being immersed in water, but it's hard to operate on both sides of the gate. Ideally I want a lock I can lock behind me when I go out and can use a key on either side and will stand the flooding. It's not a serious security issue, just a bit of a deterrent, as anybody really wanting to get in will vault the fence. This is about 4 feet high.
Any suggestions?
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