I am using an ipad from time to time next to the garmin gps 620 plotter on board and have it in a waterproof / rather shockproof case on the console of my boat. The case is from aryca and the are waterproof / solid material with a twistlock access and a thin silicone film top so you can actually use it as well inthe casing. My iphone learned to swim in one of these cases (same brand), however havent tried the ipad like this ;-).
Software : navionics is in my opinion far ahead of the competition when it comes to useability. I Tried imray as well but found it much more complicated. A plotter like a garmin 620 or 7something works simply better because the are designed for the circumstances and an iphone or ipad for that reason never will be an as good substitute.