The P38 is an awesome off-roader, bring it and it'll open your eyes as to its untapped capabilities.
As for water problems it annoys me when I see this thrown at the P38, if water is getting at the BECM then you've got so many other issues, including your driving skills, that it'll be the least of your troubles.
That goes for all other modern off roaders too. Think of how many electric motors and associated ECU's are in any luxury off-roader. To go any deeper than the manufacturers recommendation (top of the tires) requires proper preparation (axle breathers, gearbox breathers, engine air intake in any vehicle- all of which bar air intake are better prepared as standard with the P38 than any other luxury off roader I know of). If you're planning on going deep then you need to prepare for getting stuck in deep water. With the P38 this will require moving the bloody BECM and other floor mounted computers.