Yes, radio controlled boats do work on the water. In fact there are some boats that can reach over 140mph. The radio controlled boats are engineered to work great on water.
Radio controlled boats are safe electronic toys to play with in the water.
Depending on the current laws in your area, you may be limited to certain areas for using radio controlled boats. However, most areas will allow the use of RC boats in any pond, lake, or similar body of water as long as it does not endanger anyone else making use of the water.
A whistle
Ships, boats, anything buoyant, really...
That would be a hovercraft. However, hovercraft are not really boats and can travel on both land and water.
Why do motor boats spit out water
Yes, amazon does sell toy boats for children to play in the bathtub ,or any type of water. speaking from experience buying toy boats as gifts to children, they really like them.
Water
The water is controlled by dikes and ditches.
The Earth's water cycle is controlled by the sun.
Yes, salt water is denser than fresh water so boats float higher in salt water.
Jet drives are used commonly for boats designed for shallow water conditions. Jet boats can have inboard or outboard jet drives.