You are getting microphone feedback. Your mic volume is too high. Just like at a concert and the singer gets too close to a speaker and you hear a high pitched sound. Your mic is too close to a speaker and/or too high on the volume. Some boats with high towers can get 3/4 of volume, some with low towers can get 1/4. Depends on the boat. Also, some boats mic can crank when sitting but will feedback if you stand up since you got closer to the speaker.
Best thing to do is find that max set point. Test the mic at one volume and see, turn it up a bit and see, turn it up a bit more and see. Until you reach the max set point on your system. That will be the max you can get based on your speaker locations and your system.
You can also add an extension cable to the mic. So the mic is in a location where you like it and no feedback. We have wake schools that have the mics in the very back of the boat so the instructor can sit all the way back and talk.