I was viewing O.C. 321 and there are a few halachot at the beginning that mentions that one cannot salt or put cabbage in vinegar on Shabbat because it is similar to cooking. My understanding of this rule is that it seems to be in terms of preserving the vegetable for later. There seem to be permission to salt vegetables that will be eaten immediately.
Ceviche is a dish made by marinating raw fish in lemon or lime juice, mainly. If the ceviche will be eaten for a Shabbat meal, may one marinate the fish? Are the parameters of the marinating prohibition based on either the type of food being marinated (i.e. are veggies a problem but fish is not?) or is it a "time" concern (i.e., it's not a problem if you are eating the food immediately.)