Deeper the better, unless there is a safety issue. Also,
you can work in a good slope to the bottom drain.

I would argue against "deeper is better". My biggest and most recent pond is almost six feet deep. It seemed like a good idea at the time, but I now think it is too deep. The problem is I can't retrieve stuff from the bottom of the pond without donning snorkel and mask. A couple of large rocks fell off the edge of the pond into the abyss and they are still there because I haven't been motivated to dive for them. If my pond was four feet deep I could have and would have retrieved them.

I think if I was doing it again, I would make most of the pond 2-2.5 feet deep with a small section around four feet.

If you live in a severe winter area maybe deeper would be better, otherwise don't go overboard unless you like snorkeling.

I was never thinking about 6 feet deep, but 4. But I was thinking about making the whole thing 4 feet. But now that you mention having to retrieve things I think that just a small section at 4 feet for them to overwinter would be better. I can not remember how many times I had to retrieve plants and rocks that I knocked into the pond over the years. Not to mention pulling up the submersible filter to clean. Also getting the lilies at 3 feet was a chore for me.

4' is optimal for Koi. They need to swim up and down to build their muscles properly.

If you aren't keeping Koi, 3' is fine.

You can set pots of waterlilies on top of black milk crates. Gives fish a place to hide and raises them up.

No Koi only goldfish.

Like the milk crate idea. I have some small goldfish that could pass through the holes but my larger ones would not. What is the best way to enlarge the holes without leaving jagged edges that could injure the fish.

Aren't cormorants great swimmers anyway. They can dive and swim down pretty far, so I'd think of another way to beat them as opposed to going deeper.

My pond is 4.5 feet at its deepest.
I bought black utility shelves from HD to put my lilies on.
Last summer my husband bought me chest high waders which I use when I need to access the deepest part.

My hubby has a very large pair of "dykes" he say's I'm not supposed to use that word though. I used them to cut the plastic very close to the area where pieces join. I also used them to cut some much heavier plastic and though hard on the hand it worked. Glenda

I agree about the 4' depth. Our pond is 30" in about half and 4' the other half where the bottom drain is located. I should have defined "the deeper, the better". It really depends on the individual situation. I would suggest an external pump rather than submersible.

Glenda we will call them diaganol pliers, thanks for the suggestion will give that a try.

Cormorants are great swimmers, I do not know how deep they go that is why I will put net on. But was hoping that once I put bridge up, it needs work since the pond will be wider and the water lilies in they will be able to stay out of sight and no need for net. If I add a few milk crates that would give them a place to go if they get scared for added protection. Whatever went after my fish in the 35 gallon pond disappeared once the sand was replaced in backyard and since the sides are straight down I am not worried about other predators then the cormorants.

If you sit the milk crate on it's side with a flat rock on top to hold it down the fish will have a cave to hide in. The rock hides the milk crate and gives you a shelf for your plant. If you are worried about the milk crate damaging the liner you could put another flat rock, extra liner or even a couple of bricks under the milk crate. I had to use some bricks under my crates as my pond bottom is bowl shaped for the bd--which is a pain when trying to get plants to sit level! I have several milk crates in my pond and the fish love them.