Great, thanks for the advice. I was wondering if I should put some extra protection on the electronics but wasn't sure how to do this. But your suggestion is exactly what I need. Not surfing in salt water frequently, but it happens sometimes.
Regarding the print: this is just a simple PLA print which I sprayed a thick layer of paint on. Some of the difficult parts, where the paint couldn't get, are covered with epoxy glue. The screws are sealed with a small rubber washer (created from the inner tube of a bicycle tire) and the orange seal is just an elastic band which came with the broccoli from the supermarket

All in all, some fiddling but it seems to hold up.
It's not as fancy and compact as some of the other devices posted here, but it works. During designing I got really annoyed by the SD-card which takes a lot of space en therefore results in a larger housing. But all this extra air provides a nice benefit; it floats :) So in case it rips of my arm during a crash, there is a big chance I'll find it.
For those interested, I've created this with Dune3D. A very nice parametric modeler: simple yet powerful. And of course I can share the model too if anyone want to print it.