NE, Iowa??? Was the motor sold new in your area?
They install the jets at the factory and are supposed to jet according to the elevation.
But I don't really believe that??
Any idea what jet is in there now?(both carbs)
Some carbs have a high and a low speed jet(the low can be covered with a plastic plug)
How does it run at high speed?
Test: Get the motor running, go for a ride.
Then run the boat at wide open for about 10 min.
Then at full speed turn the key off.
Don't slow down or pull the throttle back, just turn off the key.
Be careful around other boats(had to say that).
Then pull the plugs and check the burn.
Post pics if possible.
Test: get the motor warm, can of starting fluid.
Spray the sf around the intake side of the motor.
Any change in the way it runs??
Yes? then it's sucking air someplace.
Sometimes a small backfire or even a cough or stutter can blow one of the port cover gaskets.
That allows air in and it won't run right?
Timing: check the timing.
Set the air screws at 1 1/2 turn out and adjust in the water, motor warm and in gear.
Hope something here helps.