Does 溺水 only mean "to drown" in relation to water? Can I use "溺水" for liquids other than water? How would you say: he drowned in Coca-cola?
Although you can not say 溺可乐, you can say 溺死在可乐中, which is more romantic anyway.

溺于可乐 is acceptable, because now it is not a fixed word but a sentence.