what island is after underwater city

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.