Fishing has been around in South America as long as there have been people living there. It was of course harder to obtain fish and seafood in some of the more remote places in the Andes, but all coastal communities and those living around lakes fished to supplement their usual diets of potatoes, amaranth, llama, alpaca, quinoa, and other foods.

There are accounts of kings themselves considering seafood a delicacy, because of its rarity in the Andean highlands. They would have it brought in on the network of roads that covered the entire empire.

Also, some communities under the Inka were allowed to pay their tribute to the empire in fish.

Sources:

Malpass, Michael Andrew. Daily Life in the Inca Empire. 1996. (44)

Sandweiss, Daniel H. Early Fishing Societies in Western South America. (Chapter from The Handbook of South American Archaeology 2008, pp 145-156)

Dixon, E. James. Bones, Boats, and Bison. 1999 (107)

I'm not sure about the Inca specifically. They were based in the Cuzco basin high in the mountains and far inland, and it would not be particularly safe to get seafood to them (I don't even eat any fish other than trout when I'm in the Andes highlands. But food safety in Peru is not exactly on par with North America or Europe). The period of Inca expansion (i.e. the Inca empire) basically conquered individual societies throughout Peru, Bolivia, and beyond, but not a heck of a lot changed in the day-to-day life of the conquered peoples and they probably kept eating what they had done before (though farming practices did change quite a bit because the Inca were basically exacting tribute in the form of crops and labour).

So that's the Inca. The Pacific Coast off of Peru is one of the richest fisheries in the world. Certainly before about 1800 BC the coast and coastal fisheries were massively important to Andean societies on the coast. In fact, the first domesticated crops in the Andean region were cotton and gourds, industrial crops used for making fishing nets and floats for the nets. Michael Moseley argued in 1974 (The Maritime Foundations of Andean Civilization) that fishing and the coast was basically the most important development in the Andes. His hypothesis was far too reaching, but protein from fish was certainly important.

After about 1800 BC ceramics appeared and corn became common and focus moved inland and to terrestrial crops and llamas for protein, but fish and maritime iconography remains common in things like Moche pottery (coastal art style from about AD 1 - 800), suggesting that they still focused on the coast. Cat fish are also a really common motif in Moche art and they would come from rivers and irrigation ditches, but it's not clear if they ate them. And you find massive amounts of shell and some fish bone all over coastal sites (from all time periods, not just Moche), which certainly suggests that they were still reliant on maritime resources for a large amount of their protein needs, while also eating corn, other crops, llamas, etc. Once you move more than 20km or so inland, though, the amount of shell seen at sites drops off considerably (no source for this, just some of my observations walking over 70 or so coastal sites).

Interestingly, though, I just fell down the Google Scholar hole and downloaded like 300 recent articles on things more or less related to my research, or just interest. One article was about stable isotope research on Nazca (contemporary with Moche but on the south coast. Moche are on the north coast) bones. Stable isotope research can be useful for showing dietary patterns and they argued that the evidence showed that maritime resources were actually a minor component of the Nazca diet, even though depictions of fish and maritime life are also very common in Nazca art. I have to go through all those articles yet so I'll update this with the actual reference if I find it in my horribly disorganized pdf library.

So, Inca specifically? I don't know for certain, but by inference I would say that fish were probably not of great importance. Not to the diets of your everyday person, at least. But fish and the Peruvian Coast is massively important in the development of Andean societies writ large, though there is some indication that its importance during younger time periods (ca. 1000 BC - AD 1450) has been overplayed.