As you're aware, the “diver flag,” is red with a diagonal white stripe and in Canada it is itemized within the Private Buoy Regulations which does not have international status. Most State (U.S.) and Provincial jurisdictions (Canada) provide that, when this flag is displayed divers must stay within a certain radius of the boat (typically 100 feet) and other boats may not intrude into that area. If divers venture beyond that radius, they should take a buoy displaying the diver flag with them. There is no signal for night diving. As already pointed-out, each country is encouraged to enact regulations that closely conform to international law for inland waterways, each country is free to specify what it requires.
Whenever a vessel is restricted in her ability to maneuver due to underwater operations, such as diving, internationally recognized Navigation Rules (in Canada, the Collision Regulations) require a small vessel (under 150 gross tons) to display International Code flag A (Alpha). The vessel thereby gains a special privilege under the Rules and may not be impeded by ordinary vessels not entitled to a higher class of privilege. This flag is only for vessels and is not carried on buoys, as is the Diver Down flag. At night, three vertically arranged, all-round lights red over white over red invoke the same privilege as the flag.
In international waters, the Alpha flag is not required by maritime law. If it's flown by a vessel however, the vessel directed to keep out of the way must follow Rule 8 of the International Regulations for Preventing Collisions at Sea (Action to Avoid Collision) and Rule 16 (Action by Give-way Vessel).
