In short
The Marine Forecast is the Bureau of Meteorology's official worded Coastal Waters forecast, the same product used for boating and shipping safety, covering weather, wind, seas and swell for the wider zone off your coast.
It reads differently to the graphs you're used to, and that's by design. The wind and wave graphs are Seabreeze's own model-driven forecast, tuned for watersports and shown as a visual timeline for a specific spot. The Marine Forecast is the Bureau's plain-English forecast for the whole coastal zone, written with a wider safety margin because it has to cover everyone from a tinny to a tanker.
Seas versus swell
"Seas" describes the local, wind-driven chop right now. "Swell" describes longer, more organised waves that have travelled in from a weather system somewhere else, sometimes a very long way away. A forecast of small seas with a solid swell running is common, and means the wind might feel gentle while there's still real size in the water.
Phrases worth knowing
Wording like "becoming" or "increasing" describes how conditions change through the period, not a single fixed number, so read the whole sentence rather than just the first figure. It's written for planning a day on the water, not for picking the exact hour to launch.
Related reading
See What's the difference between Swell, Waves and Seas? and What are the Weather Warnings, and where do they come from?