If you have a heavy overhang or something like that you might consider the fiberglass driveway marker rods avail at the home improvement stores. For normal scaping the acrylic rods will be fine. Acrylic absorbs water. Remember that things weight less underwater. If the pieces joined together are strong enough on dry land they will be ok underwater.
Don't know for sure, but I have the feeling that some who have had rods break were only using the rod alone so they could take the structure apart if needed. And did not use any "glue" to help hold the structure together. Lots of stress.
I like to use rock surface = super glue gel + Apoxie Sculpt + super glue gel = rock surface. Works really well for me. And you can closely match the color with your rock as the Apoxie Sculpt comes in several colors.