about level 42-45
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.
Azumarill learn Hydro Pump at level 57.
At level 40, Kingdra is suposed to learn Hydro Pump.
Swampert does not learn Hydro Pump.
Seadra learns hydro pump at level 40.
Croconaw learns Hydro Pump at level 51.
Milotic learns Hydro Pump at Level 40.
blastoise learns hydro pump at level 44
Samurott learns Hydro Pump at level 62.
Empoleon Learns Hydro Pump At level 59/ 99Lloyd99
Marshtomp is an evolved form of Mudkip, who learns hydro pump at level 42.