Most mammals cannot drink seawater, because excreting the salt takes more water than the seawater supplies. The limit is set by how concentrated the kidney can make urine. Tammars have a long-looped renal medulla and can produce urine well above the salt concentration of seawater, which puts them on the right side of that balance.
The tolerance is not unlimited. Studies of the behaviour report it during dry periods when fresh water is unavailable, alongside continued reliance on dew, on moisture in vegetation and on efficient water conservation elsewhere in the body, including a long gut retention time. It is a survival mechanism rather than a preference.
The tammar's other claim on scientific attention is its reproduction. Females carry a diapaused embryo, arrested at about a hundred cells, which resumes development when the previous young leaves the pouch. Because the timing can be manipulated in captivity, the species has become a standard laboratory model for marsupial development and was among the first marsupials sequenced.