For many years, Merlin's where the engine of choice for Unlimited Hydroplane racing - hence the term "Thunder Boats" due to the tremendous noise they generated. However they tuned those Merlin's to within an inch of their lives, so MTBF was pretty bad.
Eventually someone figured out how to make a small turbine work in that environment - which offered equal (or greater) power, high reliability, and much lower weight, and soon all the major Hydro teams switched over to turbine power (interestingly they still call them 'Thunder Boats' - even though they don't make that sound anymore). So, all those Hydro Merlin's were surplus'ed - and again became available for aviation.