To get to the Second Sea in Blox Fruits, you need to follow a few steps. First, reach Level 700. This is a mandatory requirement to unlock the Second Sea.
Next, head to Prison Island and talk to the Military Detective. He will give you a key and ask you to defeat the Ice Admiral.
Then, go to the Frozen Village and find the Ability Teacher Cave. Use the key to unlock the wooden door and defeat the Ice Admiral inside.
After defeating the Ice Admiral, return to the Military Detective at Prison Island. Talk to him again to receive further instructions.
Finally, go to Middle Town and talk to the Experienced Captain. He will teleport you to the Second Sea. To return to the First Sea, speak to the Sea Captain in the Kingdom Of Rose.