The Seafarer's Earnings Deduction (SED) is a tricky piece of tax legislation. In fact, even HMRC has a hard time explaining what it covers. Basically, it's a way for people working on ships to claim a 100% tax reduction on their foreign earnings. It sounds great, but the taxman talks so much about who doesn't qualify for it that it's difficult to understand who does. For instance, you'd be surprised to find out how often a "ship" doesn't count as a "ship" in the taxman's eyes. If you are an offshore worker, you could qualify for a tax refund rather than SED.