The Australian sea lions used in the film are named Spud and Friday. Both reside at Sea World Australia, and shared the role of Selkie.
Mako island is nims island.
Alexander Selkirk, the real-life Scottish Royal Navy officer upon whom Daniel Defoe based his fictional castaway Robinson Crusoe, is referenced in the names of at least five character in this movie: Jack Rusoe, Nim Rusoe, Alex Rover, Alexandra Rover and Selkie, the sea lion.
The coordinates given for Nim's Island (20 degrees south, 162 degrees west) puts it in the dead center of the Cook Islands Archipelago.
Nim gives her location to Alex as 20°S, 162°W. That is just west of Cook Islands and Arutanga in the South Pacific, and about 3,000km NE of New Zealand. There's no charted island there.