Manatees, aquatic mammals inhabiting Florida' s rivers and coastal waters, swim close to the surface and are frequently killed in collisions with boats  To address the problem, boat traffic in manatee-populated waters is being required to maintain very low speeds  Unfortunately, manatees are unable to hear low-pitched sounds and a boat' s sound lowers in pitch as the boat slows
Ttherefore, this approach may in fact make things worse rather than better
The sound of a boat engine generally travels much further under water than it does through the air.