There's a warning in the manual and (I think) a decal in the back warning about spilling liquids as it can damage the electronics. A consequence of trying to keep the weight balance - the same reason the battery is back there as well. It also keeps the engine heat away from it and that helps longevity (if you don't get it wet!).
Maybe the rubber trunk liner would be a good investment for you. The one with the lips to hold any spills.
Sealing may keep moisture out, but it also traps heat in, making the design more expensive and repair/recycling more difficult and expensive as well.