I would suggest you do either one of two things...
1. Use the Cruise Line Formal Wear for the convenience of having it delivered to your cabin. That will make for one less thing to pack and they don't seem to be anymore expensive than renting elsewhere and having to pack it.
2. With a little bit of time and effort, you can use eBay purchase a tux and all the acutraments. If you go for a used tux, you'll spend about as much money as renting one time, but you'll own it.
If you do go the eBay way, here's some of the sellers whom I'm enjoyed doing business with: thetuxxman, mikedante, & cuteclothsforkids.
Cuteclothsforkids is great because they sell infant and child size tux for about $35 (they will be made from polyester compared to wool you normally get for rented, but they look just as great and you don't mind spending LESS to buy a polyester tux a child may only fit in once than to spend MORE to rent a wool tux they will for sure wear once.