Sauce – in eel sauce

Pizza sauce is, presumably, whatever tomato-based sauce you'd put on a pizza. Those tend to be essentially like smooth (not chunky), lightly seasoned spaghetti sauce. If by "plain tomato sauce" you mean a jar of tomato sauce for spaghetti, you're probably in good shape. If you mean a can of pure tomato sauce, with no other ingredients, you'll probably want to add some seasoning. Basil and oregano are good bets for herbs; garlic and onion are also common, and perhaps crushed red pepper if you want a little kick.

If you're looking for a real recipe, this isn't a recipe request site, but search engines will serve you well.

In general, slices of raw fish or other meats are called sashimi. These are often served un-sauced, but occasionally they are served brushed with any number of soy-based sauces.

From your description, you could also be describing a Japanese-inspired version of a dish very common in Hawaii called Poke. Poke is most often made with tuna, but could also be made with other fish or seafood. The other ingredients included in Poke are highly variable.