If you spend (i think) 180+ days of a tax year overseas you become exempt from paying tax.
Normally the Crew Purser has the facility to safekeep monies for you until the end of your contract. During a 6 month contract I used to save each month and then wire money back to my UK account just before I disembarked.
If you have any personal investments (ie private pension) speak to a financial advisor as there are great benefits and tax relief if you continue to invest whilst your overseas.
For the first 3 years I spent back in the UK after working on ships I had to complete a self assessment at the end of the tax year, so keep a record of your wages and contract dates as you may have to provide them. They won't claim any tax back from you, but they may need proof you were actually working abroad.