EVERYBODY pay taxes somewhere, even if you are a seaman. If not income tax, then VAT or whatever it is called in different countries. (GST in Singapore)
The exception is only the rich, who somehow always seams to have some losses the can write off against taxes.
I know that in Singapore seamen are tax free as long as they are not in country more than 182 days/year, but so is all other citizens and PRs. (Unless you have other income derived from within Singapore, that is)
Seamen who are residents in Norway have reduced tax relative to those working in country, but they still receive the benefits that becomes to all tax payers in full.
I believe non-residents working on NIS-registered ships are tax free, but if working on NOR-registered ships, incl. in foreign trade, even non-resident foreigners pay tax to Norway and receive Norwegian benefits. (Anybody know differently?)
I know that many Norwegian seamen living abroad and sailing on NIS, or foreign flag ships, still pay into the social welfare and pension system (Folketrygden) to ensure that they enjoy the benefits that comes with Norwegian citizenship.