The island that comes after Underwater City in Blox Fruits is Skylands (often reached as you progress into the upper sky area of the First Sea).

Quick Scoop: What island is after Underwater City?

In the First Sea of Blox Fruits, Underwater City is one stop on a linear progression of islands you unlock as you level up and explore.

From the game’s island list, Underwater City is followed by Upper Skylands / Skylands , which is the floating island region you access next on your journey.

In forum-style discussions and guides, players usually describe their route as:
“...Magma → Underwater City → Skylands/Upper Skylands → Fountain City...”

So if you’ve just wrapped up your time at Underwater City and are wondering where to sail or fly next, head up to Skylands / Upper Skylands to stay on the intended progression path.

TL;DR:
After Underwater City in Blox Fruits, your next main destination in the First Sea is Skylands / Upper Skylands.

Information gathered from public forums or data available on the internet and portrayed here.