My camera manual says it can take 10mm rain for about 2 minutes. Having put this to the test a few times (and surviving), I found a need to cover my lens and filters.
Trolling the web led to this link : *removed - please read the site rules. In particular rules 3-7" thanks, Admin*
I'd be concerned with condensation on a wet day but the concept is elegant and cheap. Just get some large ziplocks for your gear before you take it near the sea. And always have your gear insured (even if you think you don't).