Should you make Feraligatr forget Water Gun for Slash?

It depends. Water Gun is a reliable low-energy Water-type move, while Slash is a powerful Normal-type move that costs more energy. Feraligatr is a strong Water-type attacker, so Water Gun is a good choice for STAB (Same Type Attack Bonus). However, Slash is a stronger move in general and can be more useful against certain opponents, such as Grass- and Steel-type Pokémon.

Ultimately, the decision of whether to replace Water Gun with Slash depends on your team composition and play style. If you need a reliable Water-type move, keep Water Gun. If you want a stronger move with more coverage, consider replacing Water Gun with Slash.