I believe that Kuki is correct on the tax situation. It is the residence of the employee that determines his tax status, not where he earns his wages. Just ask any US citizens who work abroad.
When we were on Princess last year, they offered the option of charging your tips to your onboard account. While I am not sure all of the staff like the idea that the line would have a record of the exact amount of tips they received, at least Princess offers the option as a "positive" one, not a "negative" one. In other words, you, not the cruiseline, decide if this is the way you want to pay your tips. NCL's new policy is a "negative option" where you have to go to them to prevent them from charging your account.