Morning,
The external flash is in sync with the camera in Underwater External flash mode. In Land mode the shutter and external flash will no longer be in sync. To use the external flash on land, just set the camera for underwater external flash.
Technically, you can sync the ext flash to land mode but it’s much easier to keep it sync’d to ext flash mode and use that setup on land.
In Land mode, the camera uses a different flash firing profile with pre-flashes. In Ext Flash mode, the camera does not fire any pre-flashes. Because of this differing flash profile, you would need to resync the external flash every time you switch between Land and Ext Flash modes