Armin Arlert vs Sea God: who would win?

Sea God favored — Slight edge (~55%)

Armin Arlert · Attack on Titan · Titan Form

Tier
HIGH 7-C
Attack Potency
Large Town Level
Speed
Subsonic, up to Supersonic
Durability
City Block 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.