on French radio this morning the former head of the BEA confirmed that for investigation purposes they would not need to be retrieving the aircraft. Inspecting visually the cockpit (e.g. if anti- icing was on a, which air source was selected, state of the propeller, etc.). , using an ROV would be enough.
He also said that normally a GA SEP on a VFR flight plan lost at sea would not cause a major investigation and definitively not retrieving the aircraft from the sea bed.