What is surf on Pokémon red?
Surf is a Water-type move that has been around since Generation I. Up until Generation VI, it was HM03. In Generation VII, TM94 takes its place. It is called TM47 in Let’s Go, Pikachu! and Let’s Go, Eevee!. It is called TR04 in Pokémon Sword and Shield. You will find Surf inside the hut. Teach this to your Pokémon to give it a powerful Water attack that almost always hits its target and the ability to cross rivers, ponds, lakes, and even oceans. To go swimming anywhere, walk up to a shore and choose a Pokémon from the Pokémon menu that knows Surf.

https://masterygamer.com/how-to-get-surf-on-pokemon-red/