While Crunch is a powerful move that would be a good fit for Feraligatr, it wasn't added to the game until later generations.
In Pokémon Crystal, Feraligatr can learn other powerful moves like:
* Ice Fang (TM28)
* Waterfall (TM35)
* Surf (TM38)
* Dragon Claw (TM52)
These moves can still provide good offensive power for your Feraligatr!