The Tucano was never cleared for flight over the sea due to a number of reasons.
Seriously? I don't recall any similar limitation with the JP. We didn't wear immersion suits, but then again Church Fenton, Cranwell, Finningley, Leeming, Linton and Syerstonl were well inland, as were Manby and Strubby. Although Acklington was pretty close to the North Sea.
The water-activated QRB sounds interesting. Something which doesn't operate when a student is strapping in on a wet day, but operates quickly enough to release him/her quickly enough to avoid drowning must be something of a design challenge.
But I still cannot understand why the RAF didn't include the optional the M-B Mk.17 lightweight ejection seat in the user requirement for the Prefect T1.