A Society of Petroleum Engineering TIG discussion has theorized the possibility that water sprayed on the fire sunk the Deepwater Horizon rig.  However, some of the group contend, because the rig is designed to stay afloat in hurricane conditions while taking on a huge amount of water, that water sprayed from fire fighting boats wouldn't sink it.

In my opinion it was probably a combination of the heat from the fire, compromising the structural integrity, along with the intake of the fire fighting water that caused the rig to capsize and sink.

The probable cause and effect of the rig sinking should be among the many things being investigated, relative to the BP Macondo well blowout disaster and the resultant, enormous amount of, environmental damage, for the purpose of establishing better future mitigation measures.