We visited the Maritime Museum as a group of three adults, mid-week and during the afternoon. Admission fee is $15 per adult (less for concession). Recommended to allow 2-3 hours for a visit. Paid parking is available at the Museum, alternatively it's about 10-15 minutes walk from the area near Fremantle Market.
The admissions staff were friendly and welcoming and providing us with a quick overview of the Museum. Free tours are available twice per day (one morning, one afternoon).
We went for a stroll through the various galleries of the Museum, learning about the way that the sea and coast has shaped the lives of those living in WA, from first nations, pre-European explorers, European settlement, fishing industry, immigration, transport, pearling and recreation and sport (including America's Cup). Displays provide for various ages, including activities for children.
There are options to purchase additional tours (submarine) and experiences (movies). There is a cafe and gift shop.
The Maritime Museum is filled with information. The day we visited was fairly quiet; it was most enjoyable to spend some time in the Museum where it's warm and dry on a wintry day.