According to the search results, the Third Sea can be accessed by having the required level of 1500 and completing King Red Heads quest located underneath the Colosseum. However, to access the Colosseum Quest, which is extremely important as it allows access to both the Third Sea and Race V2, players need to talk to Bartilo. Therefore, the Colosseum is not the last location in Sea 1 of Blox Fruits, and players can access the Third Sea after completing the Colosseum Quest.