All cylinders filling with water

Very unlikely that an intake manifold would be so cracked or porous that it could feed coolant to all the cylinders from the front passages & provide enough flow to fill all the cylinders with water. A leaking timing cover would explain the water in the oil but not the water in the cylinders. Water flowing in the opposite direction isn't going to leak any more than it would going in the right direction.
If there is a clamping force problem(head studs/ gaskets etc) on 1 side & the same type fasteners were used it would stand to reason that it would leak on the other side as well. I'm with crackedback & moper on this one.
A few years ago I bought a Mustang with a 5.0L in it that the guy I got it from put RTV sealer on the head gaskets & it ran until it started to get hot & then it did the exact same thing as what the OP is dealing with here. The only difference is I knew what was done already.