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?