Well, Waterfall is a Physical Water-type attack which has 80 Power, 100 accuracy, and 15 PP. Aqua Jet is also a Physical Water-type attack which has 40 Power, 100 accuracy, and 20 PP.
"Waterfall deals damage and has a 20% chance of causing the target to flinch (if the target has not yet moved)."
It has a secondary effect of allowing a surfing Pokémon to climb up/down waterfalls.
"Aqua Jet deals damage and has a priority of +1."
When used as a Z-Move, Waterfall becomes Hydro Vortex with 160 Power, whereas Aqua Jet also becomes Hydro Vortex with 100 Power.
As for which move Feraligatr should acquire, I'd go with Waterfall because it has a higher power than Aqua Jet. With more power comes more damage towards your opponent.