I believe that by "sea" you mean they hunt on ice, which is quite true. They wait above a seal hole for a seal to come up and breathe. Once the seal comes up... CHOMP! However, Polar Bears DO hunt on land. When the ice starts to melt, they move to solid ground. So in fact, they do hunt on land and on the sea.
Guinea pigs in S. America are not treated as pets, instead, they are treated like how we treat cattle. That are a source of meat for the people living in S. America. But people in the U.S, however, should NOT eat guinea pigs. If they live in the U.S.