Does the boat

The phyiscs can get pretty complicated. But basically...

Well each engine has a certain output

Perhaps one 700 horsepower engine would be more or less efficient on a boat than 7 100 horsepower engines, but I would think that the major factor would be total horsepower

Though also in the water I would think that the amount of propeller and its efficiency could matter also, so perhaps 7 100 horsepower engines might be faster than one 700 horsepower engine unless the propeller was also 7 times the size with the same relative efficiency

If they were lying on deck, their extra weight would lower the hull in the water, increase the drag, and slow down the boat...

If they were adding to pushing it forward, then yes, they'd help it go faster (once you get beyond the added weight's effect).

If the total horsepower and torque of all seven was more than what it was previously, then yes, it would change the top-end speed and the zero to top-end speed elapsed time.

A friend has a boat with two 572 cubic inch Chevy engines and it is fast. I asked him about multiple outboards and he said four big outboards would pass him like he was standing still.

Some yes but not 6 times as fast and they do have a limited speed or max speed. Where adding another motor really don’t do a thing except use more gas.

Why use an outboard or even an inboard piston engine anyway. Want to go fast. Use a Gas Turbine. [media=https://youtu.be/eQJLNtU73yk]

Too many engines would or could cause more drag. But the more engines the less the others have to work to move the boat onto plane

It might change the flow of the Ocean currents

More power more speed

Uh

It will change the price of gas