1)
Why should the boats lament?
1.
Radak: They used to go to Tzor or Tzidon for trade. Now your strength, Tzor, was plundered.
2.
Malbim: After Tzor was destroyed, also Tzidon will be. Your trade cities were destroyed.
2)
Why does it mention Tarshish?
1.
Radak: Most of the boats were in Tarshish. Merchants came to there in boats to go to Tzor or Tzidon. Since they were destroyed, you lost your wages.