I want a pond but I don't know where to start.

I have a hole . About 15 feet across  and 10 to 15 feet deep. Now this hole keeps at least 5 feet of water at all times. And my ducks love it. But  I don't know what to do.  I was told ducks will pollute a pond and you can't keep fish. Should I keep my ducks out until the ecosystem is more established?  I know some water plants are starting to grow and I am adding some.  Nothing is filling this hole outside from some rain water and water coming from the bottom of the pond should I redirect a stream to fill this pond with more fresh water?   Or should I focus on plants to clean the pond ? Do I need a liner if I'm not loosing water ?

If you want a natural pond, you are probably best off leaving it to the ducks, and well, nature in general.
Nature knows what it is doing.  It will stabilize it's own ecosystem depending on what is in the environment.

Now if you want a specific purpose pond, then that is not a "natural" pond, and you would need to interfere to get it to do what you want.
For instance, if you want it to be deeper, you may need to redirect water, add clay, etc.

I want clean ish water where the animals  can jump in  and cool off . But I'm willing to let nature do its thing. Maybe just go get some local growing water plants. To help?

Cattails and rushes are good for soaking up the extra nutrients the ducks add.  Those would also provide protection for them and other animals.
I guess it really depends on the size of the pond compared to the number of ducks and other animals that may use it.  

There has to be some balance there, if there are too many ducks or too much other disturbance, then it won't naturalize and stabilize.

If you have room for 15 ft of water, and it never goes above 5 - why not collect more water? More water for the same number of ducks will also help dilute the nutrients they add.

If you don't lose water you do not need a liner.

Unless you have some planting shelves, not many plants will grow at 15 deep.

If you leave it just to nature, you might find that the natural solution is to grow lots of algae to use up the nutrients. You are better to help nature along by adding lots of plants. Assuming not too many ducks you need about 1/3 of the pond dedicated to plants to use up the nutrients and keep algae down.

Oh clarification the hole/pond has never had less then 5 feet of water even in the worst summers we have had here (I think it was driest in 70 years) the hole had a minimum of 5 feet of water  but since winter melts its been at full capacity , it's over flowing on the lowest side actually but I have a trench  and that's all good. But I am thinking of building up the lower side more  to have more water and I was considering making shelves to make the plants easier to figure out. But I'm thinking with out a source to replenish that almost 10 foot drop of water would kinda make it hard on the plants. I may have to watch what happens this summer .

How many ducks we talking here? I just had to dig out duck muck from the bottom of my rather smaller pond so...I think the amount of ducks is a factor, for sure. I'd also say if you have a lot of ducks they'll be eating the plants. I do have plants in my smaller pond but they are protected at the root, the ducks only having access to the tops. It's working out for us that way so far.

So are the bottom 5 feet of the pond the only sealed part? Does it lose water quickly when the source stops? If so I'd be sealing it. With pigs. Because I like pigs. Plus they are delicious.

Have you solved this issue yet?