Katsuya Serizawa vs Sea God: who would win?

Katsuya Serizawa favored — Slight edge (~55%)

Katsuya Serizawa · Mob Psycho 100 · Base

Tier
7-C
Attack Potency
Town Level+
Speed
Massively Hypersonic
Durability
Town Level+, up to Small Country Level

Sea God · Berserk · Base

Tier
HIGH 7-C, up to 7-B
Attack Potency
Large Town Level, up to City Level
Speed
Subsonic+
Durability
Large Town Level

A heuristic estimate from normalized VS Battles Wiki stats: tier, attack potency, speed and durability.