The Base Set, (Japanese: 第1弾スターターパック & 第1弾拡張パック 1st Starter & Expansion Pack) is the name given to the original core release of cards and Theme Decks for the Pokémon Trading Card Game. It was released in Japan on October 20, 1996, and in the United States on January 9, 1999.......The set is one of the more well-rounded sets available, with a mixed amount of Pokémon of all types, and includes only Pokémon from the original 150. Some of the more popular examples are Charizard, Blastoise, Venusaur and Pikachu.
So it seems the Blastoise line were the first Water-type Pokemon cards, in Japan and in the United States.
There were no Water-types in the first demo pack, by the way.