As your main sea-based improvement, there's not much to say about the Fishing Boats. They are a very good source of Food, but unfortunately they don't add any Production. And, unlike in Civilization V, there's no way to gain any Production from them, save the God of the Sea Pantheon. Two civilizations, Norway and the Māori, have bonuses towards them - the Māori gain +1 Food from them and trigger a Culture Bomb when building one, and Norway's Stave Church causes them to provide their home city with 1 Production. There isn't much strategy around the former, but the latter, with shrewd Holy Site placement in cities with high amounts of sea resources, can gain impressive amounts of Production.
“Behold, I am going to send for many fishermen,” declared the Lord, “and they will fish for them.” (Jeremiah 16:16) And so it has been since men first cast hooks and nets into the waters, whether from the sea shore or the river bank. Eventually, those fisher folk built boats to pursue the fish into deeper waters. A number of civilizations – from the Polynesians of the Pacific to the Norse along the North Sea – depended upon fishing for food, oil, raw materials, and trade goods. Even where agriculture was viable, fishing boats supplied essentials unavailable by any other means. From small dinghies to large commercial vessels, fishing boats operate out of every port; as of 2004 AD, there were some four million registered fishing boats. Trawlers, seiners, longliners and whalers have prowled the seas for centuries. And some folk even think of fishing as a sport...