This is from the Humminbird FAQ page:
What products can be used to clean my display?
Humminbird recommends using Windex® Vinegar Multi Surface(in the spray bottle). Do NOT use any wipes products (which are can also be available as Windex Multi-Task).
The lens contains a special coating which improves visibility. It is recommend to use the above cleaner because we have tested and proved this cleaner. It does not deteriorate the coatings on the lens.