ChickenHouse, you may be confusing the Amsterdam FIR with the North Sea Area Amsterdam (NSAA) - see AIP ENR 2.2 para 3. The NSAA starts a few miles offshore, and in certain places actually a few dozen miles offshore.
Indeed, the rules for the NSAA are stricter than for the rest of the uncontrolled FIR. One of the rules is that it is a TMZ - Transponder Mandatory Zone all the way down to sea level.
However, outside the NSAA, outside the other TMZs, outside controlled airspace and below 1200', it is still legal to fly a motorized aircraft without a serviceable mode S transponder. (See AIP GEN 1.5 para 4)