Density Maps can get displayed at any time on the MarineTraffic Live Map. The map itself is fully responsive during that time so that you can zoom in or out as you usually do and focus on the areas you are most interested in.
In general, the zoom level varies from 2 (completely zoomed out) to 19 (completely zoomed in). You can always know the zoom level that is currently active by having a look at the URL address (as shown in the following picture).
You can choose to display All Traffic Density Maps or per Vessel Type / Vessel Size. All 3 options can be displayed in zoom level 2 up to 11.
You can also combine Density Maps with real-time traffic in all these zoom levels.