Lily, for the first time in all the years we have had a water garden/ponds in our backyard, just the other day we too had a pair of mallards swimming contently around the pond.
I tend to believe it was the strong winds that day, that forced them down just to have a rest.

At first I thought how nice it would be to let them stay and nest in the yard, until they started to pull all the plants apart. Plus I soon came to my senses when my mind went back to the time, some years ago, when our daughter gave us a pair of peking ducks as a joke gift.
I had said a few times that I would like to have a duck in the yard!! Be careful what you wish for!
Anyway we had our ducks and we made them a duck pond from a kiddie pool and they were right at home. They grew very fast from eating everything in sight. I including all the worms and insects my wife collected for them everyday.

The amount of waste those two peking ducks created in the pond and yard was unbelievable. We had to clean that pond out every day it got so bad.
We ended up giving them over to the local children's petting zoo, where they joined in with all the other quackers.

If you want them to stay be prepared for lots of mess and I hope your filter system is big and in peak condition to handle all of their wastes.

My wife went out and politely gave our new visitors directions to the near by lake where the other mallards gather. They took off and we have not seen them since.
Lovely creatures, cute for sure, but messy! LOL
"Horton"

In addition to the mess they can make, wild ducks can also bring parasites to your pond. I really don't suggest you let them stay, no matter how cute they are.

They're back after I thought they had gone. They were under my bird feeder eating seed today and then under a big bush until I scared them and they flew away. By the look of the small pond, I think they were in there too. It's muddy..

I have 6 in my yard this week too. So far they seem to be swimming in the "moat" created around our field by the heavy rains last week & I suspect they're eating the stuff in the bigger puddle out further. (A few years back we even had tadpoles in that in early spring). So far they're leaving my actual pond alone, but I think that's mostly due to it being inside the invisible fence domain of the doggie.