Maybe I wasn’t clear enough concerning the niche octillery has over the other water types. As far as I can see, the competition in regards to being outclassed as a water type comes from three pokemon, ludicolo, gorebyss, and samurott. Two of these are S-rank, and one is A-rank. Before I start my argument, octillery is generally worse than all three, and is only going for C-rank. However, it is not completely outclassed. Its niche lies in obtaining many more kos against water weak or neutral enemies than its counterparts, as well as in having far fewer counters, because it hits much harder than any of them. A full health water spout hits 1.6x as hard as a modest life orb ludicolo hydro pump, 1.29x as hard as a modest specs samurott hydro pump, and 1.11x as strong as a shell-smashed gorebyss surf. Note that water spout has full accuracy unlike hydro pump. Now you are thinking to yourself, this may be true, but what does it matter? Octillery has a 45 base speed, so who cares? These pokemon care:
252+ SpA Choice Specs Octillery Water Spout vs. 252 HP / 252+ SpD Regirock: 500-590 (137.36 - 162.08%) -- guaranteed OHKO
252 SpA Life Orb Samurott Hydro Pump vs. 252 HP / 252+ SpD Regirock: 322-382 (88.46 - 104.94%) -- 31.25% chance to OHKO
252+ SpA Life Orb Ludicolo Hydro Pump vs. 252 HP / 252+ SpD Regirock: 315-374 (86.53 - 102.74%) -- 18.75% chance to OHKO
Octillery is the only one to guarantee regirock never moves.
252+ SpA Choice Specs Octillery Water Spout vs. 172 HP / 0 SpD Eelektross: 417-492 (117.79 - 138.98%) -- guaranteed OHKO
252 SpA Life Orb Samurott Hydro Pump vs. 172 HP / 0 SpD Eelektross: 270-320 (76.27 - 90.39%) -- guaranteed 2HKO
252+ SpA Life Orb Ludicolo Hydro Pump vs. 172 HP / 0 SpD Eelektross: 263-309 (74.29 - 87.28%) -- guaranteed 2HKO
Octillery is the only who doesn’t get eelectrocuted (or u-turned on to the point where reasonable priority take it down in ludicolo’s case, but whatever, eelectrocuted sounds better).
252+ SpA Choice Specs Octillery Water Spout vs. 252 HP / 4 SpD Eviolite Gurdurr: 399-471 (106.68 - 125.93%) -- guaranteed OHKO
252 SpA Life Orb Samurott Hydro Pump vs. 252 HP / 4 SpD Eviolite Gurdurr: 257-304 (68.71 - 81.28%) -- guaranteed 2HKO
252+ SpA Life Orb Ludicolo Hydro Pump vs. 252 HP / 4 SpD Eviolite Gurdurr: 251-296 (67.11 - 79.14%) -- guaranteed 2HKO
Once again, the power comes in handy (yes gurdurr can mach punch but it only does about 25%, so octillery still 2hkos with about half its health gone)
252+ SpA Choice Specs Octillery Water Spout vs. 252 HP / 252+ SpD Lickilicky: 258-304 (60.84 - 71.69%) -- guaranteed 2HKO
252 SpA Life Orb Samurott Hydro Pump vs. 252 HP / 252+ SpD Lickilicky: 168-199 (39.62 - 46.93%) -- guaranteed 3HKO
252+ SpA Life Orb Ludicolo Hydro Pump vs. 252 HP / 252+ SpD Lickilicky: 164-192 (38.67 - 45.28%) -- guaranteed 3HKO
252+ SpA Choice Specs Octillery Water Spout vs. 248 HP / 8 SpD Regice: 187-222 (51.51 - 61.15%) -- guaranteed 2HKO
252 SpA Life Orb Samurott Hydro Pump vs. 248 HP / 8 SpD Regice: 121-144 (33.33 - 39.66%) -- guaranteed 3HKO
252+ SpA Life Orb Ludicolo Hydro Pump vs. 248 HP / 8 SpD Regice: 118-140 (32.5 - 38.56%) -- 98.61% chance to 3HKO
Octillery is the only one capable of powering through these two without a struggle. Not to mention hydro pump’s chance of hitting three times in a row is about 50-50.
There are many more pokemon octillery can power through that they cannot, and while it may be a bit of a struggle to keep water spout at full power, remember that octillery only needs to find a humble niche several ranks below his fellow s-ranked water types to obtain a C-rank. Octillery is a superior wall breaker, but an inferior sweeper. Here is a list of nu pokemon octillery outspeeds and ohkos (many after stealth rocks or somewhat unreliably- I did say he works best on volt-turn) that no other water type in nu can:
240 HP / 0 SpD Musharna, 252 HP / 4 SpD Muk, 252 HP / 4 SpD Eviolite Metang, 236 HP / 0 SpD Kecleon, 0 HP / 148 SpD Eviolite Wynaut, 252 HP / 4 SpD Ampharos, 252 HP / 0 SpD Beheeyem, 252 HP / 4 SpD Swalot, 252 HP / 0 SpD Eviolite Duosion, 252 HP / 4 SpD Eviolite Gurdurr, 252 HP / 252+ SpD Shuckle, 252 HP / 252+ SpD Probopass (sturdy, but whatever, I am hoping for mold breaker octillery in gen 6), 252 HP / 0 SpD Luxray, 252 HP / 4 SpD Wormadam-S, 252 HP / 0 SpD Eelektross, 252 HP / 0 SpD Garbodor ( just barely oustpeeds if octillery is max speed and garbodor is min speed, not to be relied upon), and many more pokemon if they can be slightly injured.
It also helps that many fast water resists like leafeon can be taken out by water spout in one hit to reduce prediction samurott or ludicolo may have to go through. If you are still not convinced octillery deserves to be C-rank, then you probably never will be, but at least I can frantically scream trick room.
P.S. I also support ursaring for b-rank. Another nice aspect no one has pointed out is that it is impossible to know if ursaring is using quick feet or guts if a toxic orb is used regardless, which can make revenge killing or switching in even trickier.