I would not rule out a crash into shallow water followed by burial into mud near the point of loss of transponder, but I feel that the evidence of a left turn over the peninsular is stronger. The oil rig 'sighting' is very weak and is offset by other 'sightings' including radar in the opposite directions.
The burial scenario is so total, that unless something like fuel seeps to the surface it has very low probability of exposure and makes searching somewhat futile.
It's like the man at night who lost his wallet and was searching under a streetlight. When asked why he was searching there he answered, "because it's the only place that I'm likely to find it".
In the fire scenario, the ACARS would have had only partial disablement in the climb to account for the later pings and this is all very unlikely. But then whatever occurred is also going to be very unlikely.