First, it's not "sea pay," it's officially called sea duty allowance.
Five years or less service with a seagoing unit will get you $274/mo before taxes. It goes up after that, then again at nine years, and again, I think, at twelve.
The incentives are based on cumulative time, not consecutive. You have to be posted or TDed to a seagoing unit, and whenever you complain about something you get to hear, "That's why you get sea pay!"
You get sea duty allowance all the time, not just when you are at sea.