For my upcoming cruise it seems that upgrading before the cruise would be a better deal. The list price for unlimited internet is $29.99/day, to pre-order the upgrade would be $229.85 when discounted for the 'trade' of the free-at-sea minutes, which works out to about $15/day since it's a 15 day cruise.
I'm debating getting that though, the cruise has only 5 sea days, and 5 of the last 6 days I'll be able to access internet in the ports.
One trick you can do - wait until the second day (may also work on the 3rd day, after which you don't get credit for the unactivated free-at-sea time) to use ANY internet at all, you pay for one less day when you upgrade to unlimited.