A: No, I'm a science and technology teacher and know a little bit about this. I would not use Modge Podge. It is water soluble and will just wash off, and the dissolved glue will not be good for your critters. You really need some sort of underwater rated material that is biologically safe. Home Depot does carry some epoxy based pond liner materials that might work, but I have not played with them. You will really need to study the manufacturer's information to be sure.
A: It's non-toxic so I think your turtles would be fine. And as long as you let it thoroughly dry (15-20 minutes between coats) it should be fine in your aquarium.