I use a dive flag for one reason only.....to keep from getting ticketed/fined myself for not having one.
I don't assume that any vessel pilot on the surface will know it's meaning and/or pilot their craft appropriately.I've seen far too many boaters who simply ignore rules and regulations,were drunk,or were some combination of ignorant/stupid to put my safety in their hands.
Rules and regulations are in place for good reasons but far too many people do not comply with them.
As an example.....A few years back I put in my 9 ft inflatable at a local lake.I had a great little boat ride until I got back to the launch site.Some idiot decided to do "donuts" at high speed around the "Slow No Wake" buoy.The wake from this completely swamped my boat.Luckily I was close to shore and able to reach safety only wet and angry.I reported the incident to the authorities (local game wardens).I gave them the registration # of the boat, a description of the operator,the make,model,and license # of the truck used to put in the boat.No action was taken by the wardens so I took my complaint to headquarters in Raleigh (NC).In the end I was basically told "We didn't see it,so there is nothing we can do".
Moral of the story...Don't rely on a flag or rules and regulations to keep yourself safe.