Without hard numbers we really can only suggest possibilities. Tired motor, the wrong prop,may too little or too big,wrong gear ratio,
Throttle not opening call the way. The foam under the floor has absorbed water, The hull has rot and is compromised.
If the motor is revving as speed is slowing the prop may have a bad hub.
Take the prop off and draw a waterproof line across the hub and body of the prop.
Do a test run and see if the line is broken.
