Rays have a "Dig In" ability that makes them nearly invisible to they player. Brown rays dig in to the bottom of muddy lakes, and yellow in the bottom of sandy lakes.
See for yourself:
The yellow and brown ray share all stats except for their attack power. The brown ray has an attack power of 1, whereas the yellow ray has an attack power of 5.
I very quickly learned to avoid walking in water when it wasn't necessary, and I look more closely in shallow lakes.
Another interesting fact about rays is they have a swim speed of 3.5. They are nearly as fast as sharks, who have a swim speed of 4. This and their dig in ability makes them difficult to kill in larger bodies of water.
http://survivalcraftgame.wikia.com/wiki/Animal%27s_Behavior_list [Visit this link to see full list of animal behaviors]