well you say "safe"
there is another issue to consider. and that is that most "safe" plants may become food
while plants add to the decor and certainly the biofilter, you must realize that adult sliders havea diet comprised largely of aquatic plants (in the wild anyway)
you may be able to quell thier tendencies by keeping them well fed though. I use elodea, anacharis, duckweed, water hyacinth and salvinia as turtle food
while many people see these as "pest" plants anyway this may or may not be applealing depending upon how much you are willing to grow and use.
Most rushes and cattails will not be touched and lilies are usually eatern either
on the other hand water lettuce is chomped with gusto as are cryptocornes and wisteria
so i'd suggest either plants LOTS of plants or expecting to lose some
or using only rushes and lilies and cattails
of course sedges and ferns are nice nearby
i can't think of any documented cases of water turtles eating poisonous water plants
its more problem with tortoises and lizards, i think.