Lord Death Man
i cant read
Hydro Pump hits harder than waterfall, even after the higher attack, because of it's higher base power (110 vs 80). A lot of threats in the metagame are either extremely frail (like Hitmonlee), or have naturally lower special defense (Drapion, Tangrowth). In addition, Ice Beam is widely considered one of Sharpedo's best coverage moves, and it hits harder than Ice Fang.
Here's some relevant physical vs special calcs
I'm sure there's more, but hopefully that helps make it clear why special is the preferred set for many people. It's also worth keeping in mind that it's also really easy and somewhat popular to support special shark with nasty plot pass togetic, while the best swords dance passers just aren't as reliable.
Here's some relevant physical vs special calcs
252+ SpA Life Orb Sharpedo Hydro Pump vs. 0 HP / 0 SpD Drapion: 263-309 (93.5 - 109.9%) -- 62.5% chance to OHKO
252+ Atk Life Orb Sharpedo Waterfall vs. 0 HP / 4 Def Drapion: 164-192 (58.3 - 68.3%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Eviolite Gurdurr: 134-160 (35.8 - 42.7%) -- guaranteed 3HKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 252 HP / 4 SpD Eviolite Gurdurr: 238-281 (63.6 - 75.1%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Crunch vs. 216 HP / 200+ Def Seismitoad: 161-191 (39.7 - 47.1%) -- guaranteed 3HKO after Leftovers recovery
252+ SpA Life Orb Sharpedo Dark Pulse vs. 216 HP / 0 SpD Seismitoad: 191-226 (47.1 - 55.8%) -- 19.9% chance to 2HKO after Leftovers recovery (phys def Seis is for Tyrantrum btw)
252+ SpA Life Orb Sharpedo Hydro Pump vs. 0 HP / 0 SpD Durant: 368-434 (143.1 - 168.8%) -- guaranteed OHKO
252+ Atk Life Orb Sharpedo Waterfall vs. 0 HP / 4 Def Durant: 160-188 (62.2 - 73.1%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 0 HP / 0 Def Ambipom: 247-292 (84.8 - 100.3%) -- 6.3% chance to OHKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 0 HP / 0 SpD Ambipom: 290-343 (99.6 - 117.8%) -- 93.8% chance to OHKO
-1 252+ Atk Life Orb Sharpedo Waterfall vs. 248 HP / 232+ Def Granbull: 105-125 (27.4 - 32.6%) -- 67% chance to 4HKO after Leftovers recovery
252+ SpA Life Orb Sharpedo Hydro Pump vs. 248 HP / 0 SpD Granbull: 312-368 (81.4 - 96%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Mega Steelix: 172-203 (48.5 - 57.3%) -- 87.5% chance to 2HKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 252 HP / 252+ SpD Mega Steelix: 307-367 (86.7 - 103.6%) -- 25% chance to OHKO
252+ Atk Life Orb Sharpedo Waterfall vs. 132 HP / 0 Def Tangrowth: 73-86 (19.5 - 22.9%) -- possible 5HKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 132 HP / 0 SpD Tangrowth: 179-211 (47.8 - 56.4%) -- 85.9% chance to 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 0 HP / 4 Def Drapion: 164-192 (58.3 - 68.3%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Eviolite Gurdurr: 134-160 (35.8 - 42.7%) -- guaranteed 3HKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 252 HP / 4 SpD Eviolite Gurdurr: 238-281 (63.6 - 75.1%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Crunch vs. 216 HP / 200+ Def Seismitoad: 161-191 (39.7 - 47.1%) -- guaranteed 3HKO after Leftovers recovery
252+ SpA Life Orb Sharpedo Dark Pulse vs. 216 HP / 0 SpD Seismitoad: 191-226 (47.1 - 55.8%) -- 19.9% chance to 2HKO after Leftovers recovery (phys def Seis is for Tyrantrum btw)
252+ SpA Life Orb Sharpedo Hydro Pump vs. 0 HP / 0 SpD Durant: 368-434 (143.1 - 168.8%) -- guaranteed OHKO
252+ Atk Life Orb Sharpedo Waterfall vs. 0 HP / 4 Def Durant: 160-188 (62.2 - 73.1%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 0 HP / 0 Def Ambipom: 247-292 (84.8 - 100.3%) -- 6.3% chance to OHKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 0 HP / 0 SpD Ambipom: 290-343 (99.6 - 117.8%) -- 93.8% chance to OHKO
-1 252+ Atk Life Orb Sharpedo Waterfall vs. 248 HP / 232+ Def Granbull: 105-125 (27.4 - 32.6%) -- 67% chance to 4HKO after Leftovers recovery
252+ SpA Life Orb Sharpedo Hydro Pump vs. 248 HP / 0 SpD Granbull: 312-368 (81.4 - 96%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Mega Steelix: 172-203 (48.5 - 57.3%) -- 87.5% chance to 2HKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 252 HP / 252+ SpD Mega Steelix: 307-367 (86.7 - 103.6%) -- 25% chance to OHKO
252+ Atk Life Orb Sharpedo Waterfall vs. 132 HP / 0 Def Tangrowth: 73-86 (19.5 - 22.9%) -- possible 5HKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 132 HP / 0 SpD Tangrowth: 179-211 (47.8 - 56.4%) -- 85.9% chance to 2HKO
252+ SpA Life Orb Sharpedo Dark Pulse vs. 232 HP / 4 SpD Venusaur: 149-177 (41.5 - 49.3%) -- guaranteed 3HKO
252+ Atk Life Orb Sharpedo Crunch vs. 232 HP / 0 Def Venusaur: 207-243 (57.6 - 67.6%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Crunch vs. 4 HP / 0 Def Virizion: 116-136 (35.8 - 41.9%) -- guaranteed 3HKO
252+ SpA Life Orb Sharpedo Dark Pulse vs. 4 HP / 0 SpD Virizion: 60-71 (18.5 - 21.9%) -- possible 5HKO
252+ Atk Life Orb Sharpedo Zen Headbutt vs. 4 HP / 0 Def Virizion: 309-364 (95.3 - 112.3%) -- 68.8% chance to OHKO
252+ SpA Life Orb Sharpedo Ice Beam vs. 4 HP / 0 SpD Virizion: 182-216 (56.1 - 66.6%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 0 HP / 4 Def Hitmonlee: 290-343 (120.3 - 142.3%) -- guaranteed OHKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 0 HP / 0 SpD Hitmonlee: 191-226 (79.2 - 93.7%) -- guaranteed 2HKO
252+ SpA Life Orb Sharpedo Dark Pulse vs. 0 HP / 0 SpD Meloetta: 242-289 (70.9 - 84.7%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Crunch vs. 0 HP / 0 Def Meloetta: 439-517 (128.7 - 151.6%) -- guaranteed OHKO
252+ Atk Life Orb Sharpedo Crunch vs. 248 HP / 252+ Def Slowking: 296-351 (75.3 - 89.3%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Life Orb Sharpedo Dark Pulse vs. 248 HP / 0 SpD Slowking: 281-330 (71.5 - 83.9%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Life Orb Sharpedo Hydro Pump vs. 252 HP / 0 SpD Assault Vest Eelektross: 165-196 (44.1 - 52.4%) -- 16% chance to 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Eelektross: 212-251 (56.6 - 67.1%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Escavalier: 169-200 (49.1 - 58.1%) -- 98% chance to 2HKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 252 HP / 0 SpD Assault Vest Escavalier: 133-157 (38.6 - 45.6%) -- guaranteed 3HKO
252+ Atk Life Orb Sharpedo Crunch vs. 232 HP / 0 Def Venusaur: 207-243 (57.6 - 67.6%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Crunch vs. 4 HP / 0 Def Virizion: 116-136 (35.8 - 41.9%) -- guaranteed 3HKO
252+ SpA Life Orb Sharpedo Dark Pulse vs. 4 HP / 0 SpD Virizion: 60-71 (18.5 - 21.9%) -- possible 5HKO
252+ Atk Life Orb Sharpedo Zen Headbutt vs. 4 HP / 0 Def Virizion: 309-364 (95.3 - 112.3%) -- 68.8% chance to OHKO
252+ SpA Life Orb Sharpedo Ice Beam vs. 4 HP / 0 SpD Virizion: 182-216 (56.1 - 66.6%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 0 HP / 4 Def Hitmonlee: 290-343 (120.3 - 142.3%) -- guaranteed OHKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 0 HP / 0 SpD Hitmonlee: 191-226 (79.2 - 93.7%) -- guaranteed 2HKO
252+ SpA Life Orb Sharpedo Dark Pulse vs. 0 HP / 0 SpD Meloetta: 242-289 (70.9 - 84.7%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Crunch vs. 0 HP / 0 Def Meloetta: 439-517 (128.7 - 151.6%) -- guaranteed OHKO
252+ Atk Life Orb Sharpedo Crunch vs. 248 HP / 252+ Def Slowking: 296-351 (75.3 - 89.3%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Life Orb Sharpedo Dark Pulse vs. 248 HP / 0 SpD Slowking: 281-330 (71.5 - 83.9%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Life Orb Sharpedo Hydro Pump vs. 252 HP / 0 SpD Assault Vest Eelektross: 165-196 (44.1 - 52.4%) -- 16% chance to 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Eelektross: 212-251 (56.6 - 67.1%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Escavalier: 169-200 (49.1 - 58.1%) -- 98% chance to 2HKO
252+ SpA Life Orb Sharpedo Hydro Pump vs. 252 HP / 0 SpD Assault Vest Escavalier: 133-157 (38.6 - 45.6%) -- guaranteed 3HKO
I'm sure there's more, but hopefully that helps make it clear why special is the preferred set for many people. It's also worth keeping in mind that it's also really easy and somewhat popular to support special shark with nasty plot pass togetic, while the best swords dance passers just aren't as reliable.