"Fifteen crabs swim in the water."
Translation:Snámhann cúig phortán déag san uisce.
What is the rule on when to lenite déag? I thought it would be here because phortán was.
I don't know what the source of your confusion is - there aren't any circumstances in which uisce would cause a t-prefix on the word before it. uisce itself can get a t-prefix, but that won't affect other words.
There's also the fact that san isn't a noun, and, for the most part, you only apply a t-prefix to nouns and numbers.