Inspected or uninspected? diesel or gasoline? With a generator and charging station or without?
See, there are no easy answers to any of this, and for me to definitively say that a fire suppression system would make a boat "safe" negates the materials of construction, the route of operation, and the use of the boat.
If I were to pick a "safe" boat, it would be built of an inherently non flammable material, steel or aluminum. It would have intrinsically safe marine wiring, and SOLAS engines so even if the fuel leaked, it wouldn't spray. I would want captain and crew to have a minimum of STCW training, and maybe advanced firefighting, advanced first aid provider, and lifeboat training. I would want the boat to be equipped with an automatic fire suppression system in any space where diesel fuel is stored or used, or where electricity is made or used. I'd want a SOLAS A pack for 200% of my passenger load, enough oxygen to keep 2 folks on O2 the whole ride home, an AED, and an advanced Trauma kit. And to train my crew in the use of all of it.
Fires happen at sea, they happen on the best of boats. They happen on Navy ships, on Coast Guard cutters, and on Liveaboard dive boats. It's what happens after the fire starts that makes or breaks a situation.