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Good water-types are Tentacruel (surprisingly), Vaporeon and Kingdra. If you have someone to trade with to evolve a Seadra, use Kingdra. I'd recommend Scizor over Steelix because Steel-types already have loads of resistances so their defense is good against most Pokémon AND Scizor has AMAZING Attack, an AMAZING ability and two AMAZING moves available in HG/SS: Bug Bite and Bullet Punch.
Steelix because Scizor has a 4x weakness to fire while steelix can have earthquake to deal with fire types quick also for water tentacruel no grass weakness