I looked up the saddening accounts and photos on the media sites. Awful!
A covered slip is a wonderful thing till a fire. That said, it seems to me that uncovered wouldn't have delayed the outcome all that much. Not after the first propane tank exploded. All those houseboats were nested together almost like a row house.
Electrical issues are my usual suspects. If they were on shore tie, and a fault within a boat or marina wiring created a path to salt water, the current flow could create enough heat to kindle a fire. Amazing what investigators can determine. We'll have to see.
Please post any updates you find.