You didn't post when you're going, so perhaps this information is too late for you, but maybe others can benefit.
You really don't need to purchase wi-fi on the ship. Yes, they offer it, but you're not on the ship all that long - you're in port every day.
I took my netbook with me and there are bars everywhere that offer free wi-fi. So I just sit down, order a beer, relax, and check my email. Plenty of McDonalds or Starbucks around that offer free wi-fi as well.
We rented a car in every port, which is the ONLY way to really get out and see the islands. Shore excursions are too expensive, too short, don't go everywhere we wanted to go, and don't spend as much time as we wanted. Most ports, we only paid about $35 for a one or two day rental. In fact, in Hilo, they gave me a brand new Mustang convertible with 12 miles on the odometer for $25 for the day!! Wouldn't do it any other way, as we went to places the shore excursions don't go, plus we got to stop and eat at some awesome places for lunch. Stopped alongside the road and got my wife a beautiful lei that this little girl was making and selling on the porch of her house. Don't get experiences like that on a big bus with lots of other people being herded like cattle.
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