Toxic Water is usually a clear color and refelects whatever color the background appears to be. For example, if the background is green, then the toxic water will resemble a green-ish tint, but is the background is blue, the the water will have a blue-ish tint to it.
Thomas, Chris, John, Laura and Sarah sink and die if they enter toxic water. Grey, Paul, Jo, Sam and Team Jump also perish if they enter water, but if they obtain Blue Strips, then they are able to float on the water without fear. James usually does not need to fear toxic water due to his reversed gravity.
Claire encounters toxic water in the first level she is in. She discovers that she can float and says that it is her super power. She can carry other characters across the water on top of her.
After the Architects redisign the world, her ability can be applied to any of the grey AI's with the help of Buffing Blocks.