Can I use "溺水" for liquids other than water? How would you say: he drowned in Coca-cola?
溺 is a formal word.
淹 is a colloquial word in daily usage.
For example, 淹死；老鼠淹死在可乐里面了（for your case）；小心别被（水）淹到；淹死你！；
These are the sentences that are used often in practice.
There are some set phrases: ...被...淹了； ...让...淹了； For example, 老鼠被可乐淹了；蔬菜让水淹了；
There is another phrase 淹没 meaning overwhelm. For example, 他被淹没了 means he is overwhelmed.