Are there any regulations that requires that international airports maintain a navigation channel deep enough to have access in case of an emergency when the airport runway ends near a body of water?
Yes, there are ICAO regulations requiring water rescue facilities. For example, from New marine rescue fleet launched at Auckland Airport:
Airports which have a significant proportion of aircraft movements over water are required by the International Civil Aviation Organisation (ICAO) to provide a specialist rescue facility, capable of an effective response in a marine area around the airport. Marine rescue craft are required to be able to transport sufficient flotation equipment to an accident site to support the number of people carried in the largest aircraft.
You can find this regulation in the ICAO Aerodrome Standards document:
Emergencies in difficult environments
9.1.14 The plan shall include the ready availability of and coordination with appropriate specialist rescue services to be able to respond to emergencies where an aerodrome is located close to water and/or swampy areas and where a significant portion of approach or departure operations takes place over these areas.
9.1.15 At those aerodromes located close to water and/or swampy areas, or difficult terrain, the aerodrome emergency plan shall include the establishment, testing and assessment at regular intervals of pre-determined response for the specialist rescue services.
Note that this does not necessarily require a navigation channel, as rescue operations can be primarily provided by hovercraft (as in Auckland).