King Ahasuerus laid a tribute on the land. Darius, the son of Hystaspes, was the first to do this (Herod; 3.89); but, as the tribute had to be rearranged from time to time (ibid; 6.42), any subsequent Persian monarch who made a fresh arrangement might be said to "lay a tribute on the land."
Xerxes is not unlikely to have done so after his return from Greece, as he had lost portions of his territories. And on the islands of the sea. The Hebrew expression translated by "islands of the sea" includes maritime tracts.
Xerxes by the Greek expedition lost the islands of the AEgean, but still held certain tracts upon the coast of Europe, which were occupied for a considerable time by Persian garrisons. These would necessarily be included in any assessment that he may have made, and it is even not unlikely that Xerxes would lay his assessment on the AEgean islands, though he might not be able to collect it.