If the cowrie shells are bigger than the turtle can fit in his mouth and they are not coated with shellac or other material, and they have been boiled or bleached to remove any dead matter inside, then they would probably be safe to use. The danger I see with them is that they are hollow and waste material will accumulate inside them, making it harder to keep your water clean. Also, even a large shell could probably be broken by a playful turtle. The calcium from the shell would be good, but the sharp jagged edges of the shell might injure the turtles insides.
The idea of hiding gravel under shells or stones won't work for an adult RES as they will easily dig through the bigger stuff and kick up the smaller stuff. I wouldn't recommend it.
Sand is another issue. Someone else here tried it recently and the tank turned into a major dust/mud bowl. It was very yucky looking and took her a long time to clean it all out. Since turtles like to dig and kick things up, I don't know if there's anyway to prevent the water from getting all mucked up.
The easiest and safest thing to do is keep the bottom bare. A few large river rocks will make it look nicer. You can also get fake plants that are already attached to a fake rock or decoration and those can look pretty nice too.
I've been trying to think of some way to make a planter for live plants that will hold some kind of substrate to root the plants in but not allow the turtle to get at it and have it still look nice.
