A: Good point, Chief. Slavery is a bit of a harsh word.
How about we call it, Life Long Employment With All Pay Deferred. That should make the matter more appealing to your tribesman.
As an added bonus, they'll get to be in the sun, all day long. Now that I think about it, I realize that's not much of a selling point. Seems like you already have a lot of that here.
But they will get to ride on a boat. Tell them that. Free Boat Ride! Followed by Life Long Employment With All Pay Deferred. Very Small Chance of Death.
You're right, probably best not to mention death at all. My math may be a bit off anyway. Wouldn't want anyone to get disappointed.
Anyway, Chief, these are your people. You know best how to sell them on the idea.
If they ask about a time table for deferred payment, mumble something about eternal riches in the next life. We find that answer works wonders.