Ventus - your water spill theory sounds possible, with reservations.
A water spill like the QF2 747 water spill would have initially caused numerous problems. You can easily imagine the pilots rerouting to Langkawi. But why wouldn't they transmit a mayday? Maybe they were too busy flying and fault diagnosing? Seems unlikely (but not impossible - after all, the 747 water spill crew didn't declare a mayday either).
Anyway, after 30 minutes the plane battery runs out and they lose all communication and navigation and many other critical systems. At this point they cannot tell anyone what is happening and they would be flying blind in a crippled plane at night with only basic flight controls. At this point they are in real trouble.
It does seem possible that in this scenario (especially if its cloudy) they get disoriented or accidentally induce a turn, and head south, with occasional changes in altitude and direction, hoping to find a runway somewhere. Their situation would get worse and worse, and eventually they run out of fuel and get forced to ditch. But would they really end up getting THAT lost?