OK - here's the short story. Here in the Chicagoland winters, we have many many ponds that are 24 inches deep. The ponds ice over, yes, but they don't freeze to the bottom. (These are in-ground ponds, not above ground.) Koi and goldfish live in them year round with no issue. The secret is to keep the gas exchange going, either by keeping a hole in the ice with a de-icer or other such device or keeping the falls running. If you don't provide for the gas exchange, the fish will suffocate under the ice. Again - that's koi or goldfish. No clue on how native fish would adapt.
Crayfish wouldn't stand a chance against 45 mil EPDM. That stuff is tough. Will they stay in your pond? That's another story. Same issue with turtles...
You CAN keep turtles, but you have to understand that they won't just stay put. You'd have to build your pond with a design that would contain them. Are we talking aquatic or land turtles? Each have specific needs. People do keep them, but it takes knowledge of their habits and building the pond to both accommodate and keep them corralled.
@Mmathis can tell you a bit more about keeping turtles contained!