I recently had a good sighting of one of these guys, I read up on them and found some info that I found very interesting.
Marsh/Helmeted Terrapins are omnvorous, they eat almost anything, including water weed, insects and frogs. In some regions they act like crocodiles, they ambush, drown and then devour doves and sand grouse drinking at the water's edge.

The female lays her eggs (10-35) in a flask shaped pit which she digs with her hind feet in soft soil, if the ground is too hard she will urinate to soften it.