spider plants are terrestrial and won't survive underwater. However, you could float baby spider plants and they will survive. I have a potted spider and I float "baby" spider plants in a cup of water in the windowsill over the winter. They grow lots of roots but the leaves don't grow much. In the spring, I pot them in dirt and they take off.
If you float babies, they need a LOT of light. IMO, it's not worth it.
As far as being poisinous, I have no idea. Best way to avoid poison is to keep them out of the aquarium.