Most electronic products for bicycles can handle some water. Even my cheap bicycle computer can handle water, but unfortunately the water does short-circuit the weak signal from the sensor, and my speed reads zero.Since I do ride in the rain, and I like to track my mileage, this was a problem.
The friendly people over at BikeForums had several suggestions. The idea that I used was to wrap a sandwich bag around the computer, then use several zip-ties to hold it in place. It’s not pretty, but it’s functional, and it was free.
I’ve done this with both of my bikes now, but here’s a picture of it on my recumbent.
It does have some drawbacks. I can’t take the computer off without cutting the zip-ties. There is a layer of plastic to look through to see the display. Overall, it seems worth it though.