"Here's what we use for non-slip stairs in a food manufacturing plant, where the surfaces are perpetually wet (and not just wet, often wet with cleaning agents that makes things slipperier.https://www.slipnot.com/products/stair-treads-and-covers/ They also make nosings to put at the front of an existing tread - our main entry stairway is porcelain tile with SlipNot nosings mortared in.  "