From the picture of the Sea Scout motor installation it might be that the motor and prop shaft are not exactly in line. Make sure the propellor shaft is well lubricated and the bearings are not worn out. Run the boat in the bathtub and feel for vibration in the motor. I suspect the flexible rubber mounting may be causing some of the problems.

@impartit
I have also found motors to be noisy on the bench, but when you put the boat in the water it doesn't sound so loud and sometimes even like a real engine. So I don't fuss about it too much, but I have picked up that the propeller shaft is the actual source of the noise and not the motor. Disconnect the coupling and rev it up again. See if the noise is less. A slight imbalance on the shaft, amplified by the wooden hull like a guitar, make it sound a lot worse than it is.
You can also try a softer rubber for the mounting. I don't use a metal mounting. I make my own wooden ones out of super-wood. So I cannot comment on the role of the mounting in the reverberation of the noise. And as a last resort, line the hull with "mouse pad" foam. Not sure what it's called but here locally we buy it in A2 size sheets. It has some sound damping properties.

I have built the Aerokits Sea Scout but the electric motor is noisy. How could I reduce the vibration noise differently from its current crude mounting on a solid hard rubber block? I am thinking of using four coil springs betwwen the rubber and the motor mount but would appreciate members suggestions. Voltage is 7.2v and PWM control from Arduino Uno with

https://www.cytron.io/p-10amp-7v-30v-dc-motor-driver-shield-for-arduino

I think the Sea Queen was about 46 inches - it was the Sea Commander that was I think 34 inches. When originally introduced, the choice was generally to power the models with a diesel of 2.5/3.5 for the Commander or an ED Viking 5cc for the Queen. Electric choice was usually a big old Taycol open-frame motor and Lead Acid cells. How times have changed.

I respect the kit manufacturers that have revived some of these designs but the construction has been changed to suit modern production methods and materials and to me they are just not the same.

@Razor. Yes. The Rover is 29". I also have an incomplete Sea Queen. The SQ is 36". They also made a larger scale model of the Sea Queen which was 42" I think. That one I sold and its now so under powered, a frog on a pedal-ski is faster 😀

I never built a Sea Rover but I had the Sea Urchin, Sea Scout and Sea Commander - all with similar construction. Lovely models although when I got divorced the part-built Commander was left behind and I discovered she thought it was "broken" and it ended up in a skip! I was heartbroken - still am. 😭
Sea Rover is about 29" length I think? Will go well with modern electrics. Good luck.

On a lighter note today...
I just picked up a donation to the club.
It's a partly built Aerokits Sea Rover.
The kit still has the original nitro engine in perfect nick.
Unfortunately, I cannot restore it to the original form, and will have to convert it to electric.
When I do get started with it in about 3 week's time, I will share the build on a new blog.
Attached however is a photo of a Sea Rover.