sorry to hear about this and hope everyone gets better soon.
Being an ex fireman and also owning a boat repair business in the Florida keys for 7 years I can tell you a lot about boats. I have refueled thousands of boats and rebuilt from bottom up hundreds. Refueling the boat may or may not have caused the explosion and here is why.
Many people leave their boats running while they refuel especially the bigger boats and that is unlikly to cause this but can if the filler tubes that take the gas from the filler to the tank are leaking.
When refueling, many times people will start their boat and get the engines warm before refueling is finished and whenever you start a inboard or inboard outboard engine you should always turn on the electric blower fans that remove the gas fumes from the engine compartment and many people forget to do this and even more of the time they just don't work right. There are so many things that can go wrong and do go wrong with boats all the time and that's why the boating repair industry makes so much money. I was in the keys last week and those guys are getting up to 150 an hour. This was most likely not user error and I hope that the owner knows that.
