Okay, well I had a scratched lens on my S4 Mini so I bought a new one for maybe £2, opened my phone up and fitted it. Not expensive and doesn't take much time (at least on my S4 mini) so if you want to see if it's the camera itself thats the problem then that's what to do. Just search "Camera Samsung J1 *your version e.g. 2016* " (make sure it's the correct year eg. 2016) and find one to order. The other possibility is that something has gone wrong in the Android Kernel and re-flashing a version of Android (perhaps an older one) might solve the issue. I don't know if you are familiar with flashing ROMs but there are guides and it is the other likely solution.

Which phone is it? The only thing you can do with a blank screen is try a hard reset (search for how to do that on your phone). Check your warranty for what the conditions are for you to have it repaired or replaced and also for what level of water resistance it actually offers. If you were within the limits it should be no problem. Basically whatever happens it will need to be examined for water damage by a repair technician to find out what parts have been affected. To be quite honest I'd never trust phones + water no matter what the advertising says!

When you say a hard reset, did you try a factory reset? That's the first step to make sure it's not software related (hopefully it is).