I'm not sure I understand if this is a question, a statement or asking if there is a problem. Let me break down what I think you are saying.
1. You turn on the AC and you get a reading of 91. This indicates incoming air temp of 91 and if the AC is in a warm location is exactly normal.
2. After the fan kicks in, soon the temp displayed shows about 81. This is indicating room temp to be 81 degrees, the ten degree drop may be simply from pulling in cooler air into the ac compartment where the thermostat sensor is., This is completely normal.
3. After hitting the set point of 76 degrees, the compressor
stops, but the fan continues to run. Again, correct operation, unlike a house AC, a marine
AC's fan runs continously.
I assume this is because of the way the thermostat works, a house AC thermostat measures the temp inside of the room, temp sensor is in the thermostat itself.
But a Marine
AC measures the temp of the incoming air into the AC unit, the temp sensor is close to the AC unit, not in the box that you think of and call a thermostat, so the only way to measure the air in the room is to have a constant flow.
If I understand correctly, it sounds as if your AC unit is operating correctly