I went through the same exercise last season and could not find an off the shelf product that I really liked. There is a lot of stuff available for phones, but not as much for tablets. The tablet stuff I did find I wasn't sure how corrosion resistant it was or it was bulky. I ended up putting together my own contraption, I will post some pictures up later when I am home. I have a nexus 7 I use for navigation (memory maps is my favorite app) as well as music. The dash on my 19' bowrider is pretty cramped. My dash never gets any spray but since no one makes a waterproof case for my tablet I wanted a mount that could hold the tablet with or with being in a waterproof sleeve (or Ziploc).
I started off with a small plastic cutting board and put machine screws through it, with about 1.25" protruding out one side. I took 3 rubber conical plugs each about 1" long and 1/2" in diameter, drilled holes down the center of each and slipped them over the three machine screws and secured them with washers and self locking screws.

I attached the cutting board to my dash with a steelie mount (brand name of mount I used) which is a magnetic cell phone mount that you can rotate and tilt around. I wanted to be able to easily change orientation and remove the whole thing quickly and easily. I also didn't want to put any holes in my dash in case it didn't work out, and this mount just uses a self adhesive material. The mount is usually strong enough, but if I slam through a big wave it will pop loose, this season I will probably add a second mount on the dash, and two more mounts on back of the cutting board so I can attach it in two orientations. My cutting board contraption allows me enough flexibility to put the tablet inside of a Ziploc if rain is expected and I can still reach all of my buttons. If a life-proof case was available for my tablet it would be perfect and my home made mount could be skipped. One handy thing I like about the steely mount is that the mount will stick to any ferrous flat surface like my fridge or range hood since the magnetic 1/2 of the mount stays with the phone/tablet. I'm sure there many other ways to go about it. A RAM mount would probably work best but they are bulky and awkward to me.