Personally having a bare bottom tank is much more easier to clean. Your turtle will do fine without anything on the bottom. But if you must, go with river rocks (bigger than your turtles head, NEVER USE SMALL FISH GRAVEL) you can find a bag for a couple of dollars at a petshop or dollar store. I believe they also sell sea shells for fish tanks (obviously if its for fish, its treated to eliminate anything bad, so I would go with that instead of picking up rocks/seashells from outside, much safer IMO) I would also worry about the sea shells being sharp, i have seen some that are pretty sharp - i can imagine a turtle poking an eye out..so watch out for that.
To round things up, i'd completely say no to rocks/seashells (waste of money, instead try buying plants, fake or real - your turtle will benefit from it (by hiding, it will probably reduce stress and if the plants are real, your turtle will get a nutritious meal out of it

) and your tank will look pretty) But like i said, if you realy want rocks or sea shells, buy some made for aquariums and make sure that the rocks are bigger than your turtles head, NOT glass, and the sea shells arent sharp.
Hope this helped
