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.
In Pokémon Yellow, Poliwag learns Hydro Pump at Level 45.
about level 42-45
Poliwag can learn Water Gun at level 13.
Swampert does not learn Hydro Pump.
Feraligatr can learn Hydro Pump at level 58 in Pokemon Crystal.
54
Lv.40
Feraligatr learns Hydro Pump at Lv. 63.
poliwag evolves at leve 25 into poliwhirl..veronica
You cannot find hydro pump. The reason is because it is not a TM. Most water Pokemon can learn it when they have reach a certain level.
24
level up a poliwag