Choice Specs Jolteon is an amazing counter to any bulky water, even specially defensive. If you run Thunder during rain Jolteon is even better. Here is some calculations to back up my opinion under rain.
Specially Defensive Jellicent 252 SpA Choice Specs Jolteon Thunder vs. 252 HP / 220+ SpD Jellicent: 372-440 (92.07 - 108.91%) -- 56.25% chance to OHKO
Wish Support Vaporeon 252 SpA Choice Specs Jolteon Thunder vs. 248 HP / 0 SpD Vaporeon: 546-644 (117.92 - 139.09%) -- guaranteed OHKO
OU Support Tentacruel 252 SpA Choice Specs Jolteon Thunder vs. 252 HP / 0 SpD Tentacruel: 446-528 (122.52 - 145.05%) -- guaranteed OHKO
Specially Defensive Rotom-W 252 SpA Choice Specs Jolteon Thunder vs. 248 HP / 232+ SpD Rotom-W: 183-216 (60.39 - 71.28%) -- guaranteed 2HKO
The 4 most common bulky waters in OU. If you are worried about things like Gastrodon, Quagsire and Swampert, you could run HP grass. 6.25 OHKO chance for Gastro, guaranteed for Quagsire and for Swampert, guaranteed OHKO again.
The options on the other post are all great as well, and just pick whichever one suits your team and needs best. Hope I helped :)