I am going to assume the engines themselves are the most expensive part and would require reconditioning regardless of landings on water or earth, why not just make that part recoverable? Parachute it back?

Can the method used by Scaled Composites, jet lifting a rocket to higher altitude, be employed as a better solution? Why not rocket power the plane portion and go for even higher based launch? Granted some cargo would be lost to weight for control surfaces and wings but recovery should be simpler, a glide path cannot be as hard as landing upright.