I have no experience with this type of piping, but from your link the specifications say the piping may be used for potable water and compressed gasses, as well as culvert piping, so it should certainly be water tight.

The installation instructions (pdf) says the couplings can be installed with or without a gasket. I'll assume the gasket is required for a watertight connection.

If you are planning on using the 90 degree fitting in a unearthed portion of the piping, such as turned upwards as a water intake on the pond side, I'd suggest the addition of some external bracing to counteract the weight and the force of the water upon the couplings.