In general how do aquatic animals address the ammonia problem

  By diffusion across their body surfaces or gill membranes.

Lysozyme - Chemical barrier;
Lysozyme is a substance found in saliva, tears, and mucus that destroys the cell wall of bacteria
Skin- physical barrier
The skin has closely packed cells and it is  tough, flexible and also water proof. 
Phagocytes -Biological barriers;
They are types of white blood cells that help protect against pathogens. 
Langerhans cells- Biological barriers;
These are specialized cells in the skin that identify pathogens that enter the body and carry them to the lymph nodes for destruction. 
Acid and Oil (skin secretions) - Chemical barriers;
These are secretions from the skin that help to keep microorganism from entering the body
Sneezing and coughing - Physical barriers.
Type of reflexes that helps keep microbes and pathogens from gaining access to the body systems.