That a lab or any other dog for that matter, that doesn't have any formal duck hunting training, could actually be a good duck retriever anyway?
What I mean is, my lab is great at retrieving bumpers and just about anything else. If he sees it hit water/ground, he's going get it. I have even thrown bumpers/balls that he didn't see and he'd sniff them out. I'll go outside and he'll want to play. I tell him, get the ball/bumper, and he'll go find it and bring it back.
I feel that if he saw a bird hit the water, he'd get it.

Outside at the house when we are on the front porch, there are 12 or so mallards(pets) someone bought and let go in the lake (more like a large pond). When they are quacking and carrying on, he's looking at searching for them. He even tries chasing the whiteling ducks when they fly over the yard.

Just wondering if any of you ever had a dog that hasn't been trained, just take to retrieving ducks like it's been doing it all it's life.

Instincts take over... I'd be taking wings & strapping them to bumpers & squirting duck scent all over the bumpers. If he's still picking them up & finding them then your onto something.

Having your dog pick up live birds (pigeons with clipped wings) and not killing them is another art too.

My one dog never made it through force fetch so I gave up on her & she's just an expensive house pet.

Yeah both my dogs never had any formal training other than basic sit, heel, and drop commands.
I wish I had the time to do formal training, but they go above my expectations for the lack of training they had.
They do well, they rely on sight and smell.
If a bird goes down in the marsh, my oldest will run zip zags in the marsh till she finds the bird.
Instincts tend to take over and bringing out there at an early age lets they become use to doing it.