Carseats?

We are taking a cruise to Hawaii over christmas (yeah, I have a few months to plan Wink ). Our kids will be 2 and 3 years old. I was wondering about car seats for being off the ship. I cant imagine carrying around 2 car seats, but i know there are car seat laws...what does everyone do? I was thinking we could just rent cars in each port with car seats. Has anyone done that?