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.
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.
No, you can't. For people whom are unfortunate and stupid to be deceased causing by coca-cola, it's called 食物中毒, food poisoning.
他掉进了一大缸可口可乐里面溺死了。 He fell in a big vat of coca-cola and drowned.
(That should make the cola taste piquant.)