The easiest way to test if your RAI is sealed is to take the top off it and place a piece of wood over it with the engine running. If the engine stops then it is sealed, more than likely it will not stop unless you have sealed both of the drains in the air box and replaced all of the hoses between the RAI and the air box with something like the nugget stuff kit.
In a stock defender at 700mm as others have said you do risk water reaching part of the air system, especially if its a long crossing.
Have you also replaced your fuel and diff breathers? If not then personally I would not suggest it being a good idea.

