The IMO (International Maritime Organization) states that "Ships fitted with AIS shall keep their AIS in operation at all times, except where international agreements, rules or standards provide for the protection of navigational data."
Clearly, if you have an AIS transponder on board, you must keep it on. There is no question of turning it off so that it cannot be seen by others. Indeed, it is an obligation of the SOLAS Convention (regulation 19.2). If this convention is primarily intended for large vessels (cargo ships, container ships, oil tankers, liners, bulk carriers...), yachtsmen are also concerned by these injunctions.
A Class B AIS transponder found on pleasure boats transmits the boat's identification number, name, course and speed. It is therefore a very practical and reliable safety equipment. Nearby boats that receive the signal can see the route and avoid a collision. This transponder is not mandatory (but strongly recommended).
If for some unjustified reason you keep this transponder turned off, you won't risk much with the authorities. On the other hand, if you have an accident, your insurer (or that of the other party) could use this loophole to avoid covering the damage. In case of a personal accident, this can be very expensive..
So if you have an AIS transponder, don't be tempted to turn it off. Your safety is at stake.
