In addition to the same itemized deductions and tax credits that you might qualify for, the following link provides a tax work sheet especially designed for merchant marines or sailors.
Some tax deductions you might want to consider are travel expenses, union dues, uniform expenses if one is required, specialized equipment, licenses and fees, job required medical exams, etc.
The document found at the following link is provided by Martin Kapp, CPA, EA,
I recommend that you also take a look at the following IRS publication 463, Travel, Intertainment, Gift and Car expenses. It provides several rules and guidelines including limitations on which expenses you might be able to deduct.