In Pokémon Yellow, Poliwag learns Hydro Pump at Level 45.
I found that poliwag learns hydro pump at level 43. it's the same for gold, silver and crystal, but you'd have to keep continually stopping poliwag from evolving as it evolves into poliwhirl at level 25 and it learns hydro pump later on, at level 48.
about level 42-45
Poliwag can learn Water Gun at level 13.
The only Pokemon that can learn Hydro Cannon are: Blastoise, Feraligatr, Marshtomp, and Empolion.
Salamence can only learn Hydro Pump as an egg move in Pokemon Emerald.
Swampert does not learn Hydro Pump.
You will have to trade or make it learn hydro pump
Feraligatr can learn Hydro Pump at level 58 in Pokemon Crystal.
swampert can not learn hydro cannon in R/E/S it can only learn hydro cannon in Pokemon pearl/platinum/diamond SEE YA
Most Water Based Pokemon and some Psychic Pokemon
Gyarados cannot learn the move Hydro Cannon. Hydro Cannon is a move that can only be learned by the Water type Starter Pokemon.
Lvl. 46 in blue/red/yellow Lvl. 49 in fire red/leaf green/ruby/sapphire/emerald And not sure about the rest... Hope this helped