King Ahasuerus imposed tax on the land and on the coastlands of the sea.
We are here told, I. How great and powerful king Ahasuerus was. He had a vast dominion, both in the continent and among the islands, from which he raised a vast revenue. Besides the usual customs which the kings of Persia exacted (Ezr 4:13), he laid an additional tribute upon his subjects, to serve for some great occasion he had for money (Est 10:1): The king laid a tribute.
Commenting on Esther 10:1-3
And the King Ahasuerus laid a tribute on the land, and upon the isles of the sea. Which include all his dominions, both on the continent, and on the sea, the Aegean sea; though Aben Ezra thinks it regards such as were not under his government, but stood in fear of him, of whom he demanded tribute.
Laid a tribute upon the land - On the one hundred and twenty-seven provinces of which we have already heard. The isles of the sea - Probably the isles of the Aegean sea, which were conquered by Darius Hystaspes. Calmet supposes that this Hystaspes is the Ahasuerus of Esther.