はこにわ大海戦 is a free-to-play, open-world naval strategy game played in a web browser on PC or smartphone, with no installation or email registration required. Inspired by the classic Hakoniwa Islands naval game, it combines island development, fleet building, and multiplayer diplomacy in a shared ocean. Players grow their islands by establishing farms and factories, construct and organize ships, and plan routes and commands in a turn-based world where careful strategy matters more than quick reflexes.

Alliances, friendly agreements, declarations of war, and ceasefires let players shape rivalries and cooperate as they compete for influence at sea. New players can use guided training to practice building facilities, commissioning ships, and entering their first battle before moving into regular play. The site also offers ocean maps, ship and aircraft catalogs, a rules guide, a wiki, discussion boards, rankings, replays, and an official Discord community.

A separate test sea lets participants try implemented vessels and upcoming features without affecting their main game. Play is free, with optional paid items available