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By leveling up, Feraligatr learns Hydro Pump at different levels for each generation.In Generation II (Gold, Silver, Crystal), Feraligatr learns Hydro Pump at level 58.In Generation III (Ruby, Sapphire, Emerald, FireRed, LeafGreen), Feraligatr learns Hydro Pump at level 55.In Pokémon Diamond and Pearl, Feraligatr learns Hydro Pump at level 63.In Pokémon Platinum, HeartGold and SoulSilver, Feraligatr learns Hydro Pump at level 71.In Generation V (Black, White), Feraligatr learns Hydro Pump at level 76.
lvl 30 or 40 something
It learns Hydro Pump at level 48.
blastoise learns hydro pump at 63
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.
Seadra learns hydro pump at level 40.
Milotic learns Hydro Pump at Level 40.
Croconaw learns Hydro Pump at level 51.
Prinplup learns Hydro Pump at level 51.
blastoise learns hydro pump at level 44
Samurott learns Hydro Pump at level 62.
Vaporeon learns Hydro Pump at level 71.