The simple answer to both of these questions is yes. But, there’s so much more to this story…
The other day I got into a somewhat heated argument with an angler who swore an idling four-stroke outboard would scare away fish if the boat drifted over them. IMHO he was dead wrong, and here’s why I say that: I’ve dropped a hydrophone beneath the water’s surface and recorded the sounds of a number of potential fish-spooking noises. Four-stroke outboards, two-stroke outboards, slamming hatches, dropped pliers, people talking, electric trolling motors, coolers dragged across the deck, and even push-poles used on mud, sand, and shell bottom. I’ve done this at varying depths (just a foot under the surface, 15′ deep, and 30′ deep) and I measured the strength of each of these noises with a dB-A meter. And truth be told an idling four-stroke is just as quiet beneath the surface as it is above it. A two-stroke is a different story, and the sputtering they make is quite audible beneath the water. But once you shift these motors into gear, their underwater sound levels are essentially equal. Now, ready for a surprise? The sound level of an electric trolling motor is right on par, because once the propeller is turning, the loudest sound any of these motors make is the whine of prop noise.
But let’s not forget that there are a number of ways boats spook fish (we talk about some in Why We’re Spooking Fish With Our Boats). And the fact of the matter is that closing a hatch or even a person speaking in a raised voice are often quite a bit louder underwater than anything the engine might be doing.
Could a boat drifting over a fish spook it with its shadow? It wouldn’t surprise me. Could someone on that boat yelling, stamping, or dropping something to the deck be responsible for scaring off the fish? Absolutely. But an idling four-stroke outboard? Nah.