Yes, however, you must apply for a special authorisation. You will find more information on the page Professional users – Mobile networks
The use of radios from the ground as an instructor or in AFIS (Aerodrome Flight Information Service) is possible but is subject to an authorisation of another category.
Both. An aeronautical authorisation is required when airborne but a maritime authorisation is required when on water. Furthermore, it can use a PLB: a device which can be activated by the user in case of emergency. That type of device is not subject to an authorisation in Belgium, provided that it is registered by the user via www.406registration.com.