I am Scuba diver also. Recommendation for Scuba dive in Asia which is not too difficult to access from Japan.
Malaysia, not the continent part but on island of Borneo on Eastern shore has very nice Scuba dive area. Flights from Narita by Malaysia Airlines. Bit more than 10 years ago there was an incident of local Indonesian militia group holding Western Scuba divers’ hostage, so I am not sure what is the situation today.
Palau, island west of Guam and Delta Airlines has non-stop flight from Narita. It is a beautiful island for Scuba diving, one of top locations for Scuba diving I have been, but not as great as Maldives.
Would you consider going all the way to Maldives? Flights by Sir Lanka Air (via Colombo), Singapore Air (via Singapore), or Korean Air (via Seoul) from Narita.
Chuuk (formerly known as Truk) on Micronesia, east of Guam. Reached by United Airlines via Guam from Narita. Highlight of Chuuk is wreck dive, there are sunken WWII Japanese batter ships and it was something to experience, for me.
These are places I enjoyed in Asia which is not that difficult to access from Japan and these locations are not very advanced Scuba locations but may not be for novice scuba divers.