KAHU aims to close a persistent gap in maritime visibility - the inability to continuously track vessels not transmitting AIS (Automatic Identification System) data. AIS can be switched off, spoofed, or absent, creating blind spots exploited by illegal fishing, shadow fleets, and other high-risk actors. The AIS market is also dominated by a few providers, limiting access and transparency. This affects not only maritime safety and security but also the monitoring of ocean environments. We address this by building the first real-time, crowdsourced radar detection network for the ocean. Our onboard software connects to existing vessel radar, collects radar echoes, and streams them securely to the cloud using low-bandwidth protocols. There, echoes are linked into vessel tracks and fused with AIS, satellite imagery, and other data layers to produce a continuous, sea-level view of vessel movements, enabling comprehensive visibility over human activity at sea and supporting timely, informed action.