Both are great options.
Swampert’s a better water attacker with hydrocannon, since Water gun’s higher energy rate, and when using what is one of the strongest moves in the game, greater frequency of usage matters a bit more than more damage on your fast move. In cases where the charge move is something like hydropump, the differences between EPS and DPS of waterfall/gun are somewhat negligible, but with a move that hat more damager per energy like hydrocannon…yeah watergun’s better. That and Swampert’s got the better attack stat, so even if it was using waterfall, it would be out damaging feraligatr.
Feraligatr was by no means a bad investment, it was through two groudon runs, and entei day as the best nonlegendary water. But most of the hype for swampert stems from PvP, where its the fastest hydrocannon spammer. Ground types are great in PvP with mudshot, but since most (ie groudon) are burdened with high cost charge moves, they are very reliant on shields being down. Swampert doesnt have that issue, able to be the shield breaker and the ‘go for the throat’ attacker at the same time, where as Groudon was dependent on either locking an electric type in, or having shields down for it.
Its kinda the dragonite/Salamence senario, Dragonite’s great, but something else is better, does that inherently make dragonite any worse of an attacker than it was? No, it still does just as much as before, and it still served well even before its competition was added.