Yes, lions will eat fish. Lions have been recorded feeding on almost anything and everything, mostly land mammals but also some aquatic species including fish, crocodiles and seals. The authorities for the fact that lions occasionally eat fish are (1) C.A.W. Guggisberg – Simba: The life of the lion published in New York by Chilton Books in 1963 and (2) G.B. Schaller in The Serengeti lion 1972, Chicago University of Chicago Press.
It is interesting that lions hunt for seals along Namibia’s Skeleton Coast. H. Berry in Namibia’s seal-eating lions in danger of 1991, states that lions walk as far as 50 kilometres a day along the beach scavenging for seal and cormorant carcasses which have been washed up on the tide. Seals are not fish but warm blooded, air-breathing mammals. But perhaps fish are occasionally washed up too presenting the lion with an easy meal. And there will be fish inside the stomachs of seals!