Presumably if you own a boat, price is no object to you and you might find purchasing a waterproof "case" at between $20 and $100 not unattractive. But frankly if you think out of the box a little, there's a solution you could use -- mount included -- that not only costs pennies, but is amazingly efficient and is something you probably already possess in your home: A full-sized Ziploc freezer bag and some Velcro strips.
Ziploc freezer bags not only offer plenty of room to fit your IPad inside, but, unlike some other cases with internal snaps or locks, installing or removing your device takes only a few seconds; also, the screen is easily viewable inside, facilitates finger manipulation (CAREful...), and the plastic is of sufficient thickness to alleviate casual tearing (remember, these are the freezer bags and not the generic sandwich bags). I use one myself when I take my IPad to my hot tub! I wouldn't recommend submersing it but for anything short of a fire hose blast of water, it laughs at all drips and drops.
As for the Velcro, just obtain two sets of matching 3" strips, place one set on your hopefully angled boat dashboard, on any other convenient surface, and the matching set on the bag and voila! There's your mount!
Here's somebody else's pic I just found on the internet of one in use for this purpose (although I turn mine 45 degrees). Hell, there's still room for a half-dozen Oreo cookies!!