What are the mechanical requirements to fly over a large water body? Are multiple engines required? Do the engine/s need to be a certain kind (turbofan, turboprop, piston)?
Private operations - no requirements. Single-engine is legal, you can argue as to the safety.
Single-engine: within gliding distance of the land.
Twin-engines: always within 60 minutes of the land... unless specially approved airplane and operator, and airplane maintained according to much higher specs - this is ETOPS.
More-than-two engines: no problem.