Before I do this list;
I know you're probably asking this question because you think not many things can tank a Specs Kyogre Water Spout in Rain, at Full Health.
Hate to break it to you.
You're wrong.
Alot of Pokemon can take a single hit from it - anything which resists Water Spout, and has a decent Special Defense + HP can tank one hit.
I listed some prominent examples below - Pokemon which might have some use in Ubers. That is, until you get to like Dragalage and Mega Blastoise which I just put in for I don't know why.
Blissey and Chansey
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Blissey in Rain: 393-463 (55 - 64.8%) -- guaranteed 2HKO after Leftovers
252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 4 SpD Eviolite Chansey in Rain: 429-505 (60.9 - 71.7%) -- guaranteed 2HKO
Lugia
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 80 SpD Multiscale Lugia in Rain: 218-257 (52.4 - 61.7%) -- 97.7% chance to 2HKO after Leftovers recovery
252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Lugia in Rain: 357-420 (85.8 - 100.9%) -- 6.3% chance to OHKO
Giratina
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Giratina in Rain: 213-252 (42.2 - 50%) -- guaranteed 3HKO after Leftovers recovery
Ludicolo
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 4 HP / 0 SpD Ludicolo in Rain: 168-198 (55.6 - 65.5%) -- guaranteed 2HKO after Leftovers recovery
Palkia
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 4 HP / 0 SpD Palkia in Rain: 143-169 (44.4 - 52.4%) -- 20.7% chance to 2HKO
Tentacruel
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 0 SpD Tentacruel in Rain: 287-338 (78.8 - 92.8%) -- guaranteed 2HKO after Black Sludge recovery
Certain variants of Zekrom
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 104 HP / 0 SpD Zekrom in Rain: 336-396 (91.5 - 107.9%) -- 50% chance to OHKO
Arceus (Types which Resist)
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 4 SpD Arceus-Grass in Rain: 286-337 (64.4 - 75.9%) -- guaranteed 2HKO
Ferrothorn
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 0 SpD Ferrothorn in Rain: 296-348 (84 - 98.8%) -- guaranteed 2HKO
Dialga
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 4 SpD Dialga in Rain: 335-394 (82.9 - 97.5%) -- guaranteed 2HKO after Leftovers recovery
Goodra
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 4 SpD Goodra in Rain: 235-277 (61.1 - 72.1%) -- guaranteed 2HKO
Kyogre
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 4 HP / 0 SpD Kyogre in Rain: 250-295 (73 - 86.2%) -- guaranteed 2HKO
Regice
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Regice in Rain: 291-343 (79.9 - 94.2%) -- guaranteed 2HKO after Leftovers recovery
Non-Water Absorb Mantine
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Mantine in Rain: 191-225 (57.1 - 67.3%) -- guaranteed 2HKO after Leftovers recovery
Milotic
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 0 SpD Milotic in Rain: 277-327 (70.3 - 82.9%) -- guaranteed 2HKO after Leftovers recovery
Dragalage
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 4 SpD Dragalge in Rain: 280-330 (83.8 - 98.8%) -- guaranteed 2HKO after Black Sludge recovery
Mega Venusaur
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 4 SpD Mega Venusaur in Rain: 286-337 (78.5 - 92.5%) -- guaranteed 2HKO
Suicune
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 0 SpD Suicune in Rain: 298-351 (73.7 - 86.8%) -- guaranteed 2HKO after Leftovers recovery
Mega Blastoise
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 4 SpD Mega Blastoise in Rain: 297-351 (82 - 96.9%) -- guaranteed 2HKO
Both Latios and Latias (With investment, or with Soul Dew)
To illustrate my point earlier, where I said alot of Pokemon can tank it, provided they Resist it and have good defenses. Here are some Pokemon which haven't evolved fully which can do it.
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Sliggoo in Rain: 222-261 (65.2 - 76.7%) -- guaranteed 2HKO
That's right. Sliggo can tank it.
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Eviolite Goomy in Rain: 195-229 (66.3 - 77.8%) -- guaranteed 2HKO
That's right. Goomy can tank it.
>252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Eviolite Squirtle in Rain: 212-250 (72.6 - 85.6%) -- guaranteed 2HKO
That's right. Squirtle can tank it.
So yeah. Unevolved Pokemon can tank it, provided they have Eviolite, resist it, and have an existant HP and SpD.
Alot of things can tank Water Spout. Just they can't tank it repeatedly.