At one point Schreiber toted Samuel on his shoulders while they splashed around in the ocean.
Baker is from the Australian state of Tasmania and his wife is from Sydney. Watts was born in England but grew up in Australia.
The Diana star was spotted on the beach with the kids yesterday, though unlike her partner she didn't strip down to her swimsuit.
Who knows how long she and the fam plan to stay Down Under, but they should be back in L.A. by Jan. 12—Schreiber is nominated for a Golden Globe for Best Performance by an Actor in a Television Series, Drama, for Ray Donovan.
Having played President Lyndon Johnson, he's also part of the Screen Actors Guild Award-nominated cast of Lee Daniels' The Butler—the SAG Awards will be handed out Jan. 18.