I prefer fishing the river. It makes more sense to fish in the ocean cuz thats where the big bells are at, but I like fishing for fishing. There's better ways to make bells.
I definitely prefer fishing in the sea, since it feels a lot faster to spawn fish by running up and down the beach, along with the fish there generally selling for more money. Plus the fish tend to be cooler with sharks and whatnot.
If on average, I would say that I like sea fishing more. However, since it is summer season for north hemisphere, there are a lot of lucrative river fishes including arowana, angelfish, dorado etc....so I probably will do river fishing mostly during summer but sea fishing more on other seasons.
Sea fishing, since others have said that you find more expensive fish there than in the river. Plus, I can run up and down the beach to gather clams as well to turn into bait and then use off my pier.