Games like Underwater World
Games like Underwater World include Hollow Knight, Rain World, Thank Goodness You're Here!, Transformice, and Hollow Knight: Silksong. We matched them by the tags players give Underwater World on Steam (Casual, Adventure, Platformer, Simulation, and Side Scroller): closest matches first, and among equally close ones the better-known games.

Hollow Knight
Shared: Platformer · 2D · Adventure

Rain World
Shared: Exploration · Platformer · Atmospheric

Thank Goodness You're Here!
Shared: Adventure · Casual · Exploration

Transformice
Shared: Casual · Platformer · 2D

Hollow Knight: Silksong
Shared: Platformer · 2D · Adventure

Home Behind
Shared: Adventure · Casual · Simulation

DAVE THE DIVER
Shared: Casual · Adventure · Exploration

The Long Dark
Shared: Exploration · Adventure · Survival

Ori and the Will of the Wisps
Shared: Platformer · Adventure · Exploration

Chained Together
Shared: Adventure · Casual · Platformer

Deadlight
Shared: Platformer · Adventure · Side Scroller

LIMBO
Shared: Platformer · Adventure · 2D

Terraria
Shared: Adventure · 2D · Survival

Slime Rancher
Shared: Adventure · Exploration · Casual

FEZ
Shared: Platformer · Adventure · Casual

Mark of the Ninja
Shared: Platformer · Adventure · 2D

GRIS
Shared: Adventure · Platformer · Atmospheric

The Forest
Shared: Adventure · Survival · Exploration

BEEP
Shared: Adventure · Platformer · 2D

Dust: An Elysian Tail
Shared: Platformer · Adventure · 2D

FAR: Lone Sails
Shared: Adventure · Exploration · Atmospheric

Sparkle 2 Evo
Shared: Casual · Simulation · Adventure

Russian Life Simulator
Shared: Simulation · 2D · Casual

Game Dev Tycoon
Shared: Simulation · Casual · 2D