The ECU itself is apparently water tight, what isn't is the connector plugs going into it. At least according to an ex LR mechanic... The suggestion was to use a silicone sealant where the wires run into the connector block.
Breathers as mentioned, Front & Rear Axles, Gearbox, Transferbox. Run them all into a multi plug in the engine bay and run one or two lines to the top of your snorkel. Also have a fuel tank breather which I ran into the rear of the interior and sealed up the hole it came through. Don't bother with buying there ridiculously expensive 'wading kits' from LR sellers or ebay, just buy the parts from an air line supplier, cost me £12 for the whole lot not £70 some charge!
I'd also remove any carpet if you have any and move anything valuable off the floor. Even with a good bow wave water still comes in!
After many years of having my Defender I wont go in muddy water anymore it just isnt worth it, wrecks so many things!