How difficult and expensive is it to muffle the sound of the motor on a boat?

Cost is the main reason. Boats rattle alot more than cars. They’re often designed to as they experience twisting and pushing forces, temperature variation, and such where I suspect being intolerant to these would probably be bad structurally. Boats are also typically designed to drain. You want the water flowing down to the lowest part of the boat (or off it) where the sump pump is. All those spaces and gaps lead to vibration.

You could probably add a ton of padding everywhere, but how well would it stand up to water? Especially sea water? I’m sure there are specialized types, but I’ll bet they’re expensive.

Was it an inboard or an outboard motor?

it is a little difficult and costs a little more. And there are no specific noise standards so there is no incentive for the makers and owners to do it. Gasoline car engines are quiet – unless the owner wants to make them loud. Look how quiet the engine is in your car. The resonator and the muffler do a fantastic job of quieting things down. Where would you put those on a boat? There should be enough space but the owners and makers don’t bother.
If people/customers demanded it there would be some incentive
Unfortunately many owners like the noise. We all know the one jerk driving his noisemaker around the quiet lake bothering the peace and quiet of hundreds of others. It is always a guy with a beer. Science.

(I know of a quiet, non-commercial, boat that actually mixes water in with the exhaust and releases the mix just under the water surface. )

There are noise limits on power boats. Depending on the State there’s noise limitations from 70 to 90 decibels ar measured from the shore. 70 decibels is the sound a washing machine makes while standing next to it.

I often go canoe paddling all year around at a near by lake in the PNW. It’s a lake that’s patrolled by police and fire boats. Yet I periodically experience power boats far exceeding the noise limits. I mean the noise can reach deafening levels.I don’t know if the police enforce the noise limits.

I can afford to buy a house on the shoreline of a lake but would never live on a lake because of the noise. On warm summer days the noise gets overwhelming with many powerful ski boat engines roaring right and left with some boats blasting music over load speakers.

I think the Universal boats are less noisy, but maybe I’m remembering incorrectly. I’ll have to check the decibels next time I’m on each of them. I don’t ride the boats often so it might be a while until I do that.

The noise ruins the ride. This is the boat if you care to see. https://youtu.be/h1VVuhc0XD4 It could be really pleasant otherwise.

Is there no such thing as an electric motor that would be quiet? We’re in the Florida sun, it could even recharge with solar (a little) through the day. Probably not practical.

They should use paddle boats. Those are very loud.

Times I’ve been on a motor boat, they’d cut the motor and we’d just be out there. The motor would get us from point A to point B but then we’d shut it off and just hang out.Then they put the motor back on and maybe we’d drive to another area, or drive around a little, and then go back to the dock.

@JLeslie A boat of that size absolutely should not be so loud. There is enough space and budget to make it quiet. If customers complained they might do something about it.