Where to Find Dewott

Generation 1

Red
Blue
Yellow

Generation 2

Gold
Silver
Crystal

Generation 3

Ruby
Sapphire
Emerald
Fire Red
Leaf Green

Generation 4

Diamond
Pearl
Platinum
Heart Gold
Soul Silver

Generation 5

Black

Evolve Oshawott

White

Evolve Oshawott

Black 2

Evolve Oshawott

White 2

Evolve Oshawott

Generation 6

X
Y
Omega Ruby

Evolve Oshawott

Alpha Sapphire

Evolve Oshawott

Generation 7

Sun

Evolve Oshawott

Moon

Evolve Oshawott

Ultra Sun
Ultra Moon
Let's Go Pikachu
Let's Go Eevee

Generation 8

Sword
Shield
Brilliant Diamond
Shining Pearl
Legends Arceus

Alabaster Icelands: Space-time distortions (after completing Mission 18), massive mass outbreaks (after completing Request 102)

Generation 9

Scarlet

The Hidden Treasures of Area Zero DLC:

Evolve Oshawott

Violet

The Hidden Treasures of Area Zero DLC:

Evolve Oshawott

Dewott Evolution Tree

Level 17

Level 36

Level up in Hisui for Hisuian Samurott