I have a duck that gets very loud when I get close. She gets very quiet when I refill and bring back the food
Ducks make all kinds of noise when they get excited, for both good and bad reasons. It's normal. Seeing you is exciting. You bring food. You fill the pool. You steal the eggs. You invade their territory. You open and close the gate. You might be carrying tools. All of these things are exciting in the world of a pet duck.
If your one duck is hogging the pool and the other doesn't ever get a turn (mine seem to take turns instead of swimming all at once), you might want to set up another water source nearby. It could be smaller. Just enough to be a bit of a distraction so they both get to get some water action. It's good for them for cleaning and preening, plus it's fun.