I have used both and they are both good.
The lifeproof is pricey but is fully (ie. immersible) waterproof but mine was not incredibly durable, only was waterproof for maybe 4 months before parts started going bad. The ports can get worn out and you can only use apple accessories since third party cables, headphones, etc. don't fit the ports.
The otterbox is good protection, no real complaints but nothing fancy.
But my favorite is the one I have for my son's tablet. My son has down syndrome and autism and is prone to fits and is generally hard on stuff (why I got the lifeproof for my iphone, toilets and iphones don't mix). I got him a gumdrop case. The custom ones are no more than the standard ones.
Gumdrop Cases - Custom Drop Tech Case for iPad 2, 3, 4 - Custom Series - Collection
I like the fact that you can get it in colors, not just Henry Ford Black. It has the same basic build as the otterbox, a hard plastic frame surrounded by a rubber outer layer, but the outer layer is thicker and much more textured so gives it a better grip and a little more protection. It has rubber flaps over the ports which provide easy access and accommodate third party cables and such. With the ports closed I would call it splash proof and wouldn't hesitate having it on the boat. It's also a good price at $60.
Really the ultimate is the lifeproof with the floating lifjacket wrap around but you are talking $117 for the case (on sale) and $45 for the lifejacket, but you could then use your ipad for man overboard drills! I'd rather buy a $60 case and put the rest of the money towards a spare refurbished ipad.