Hello, and welcome to Cruisemates!
First, you don't have to pack a life jacket. If there's not an infant jacket in your cabin when you arrive just ask your Room Steward for one.
About car seats, I'm just going by what I've heard and read, but usually
there are no seat belts in the back of taxi's so a car seat wouldn't really do you any good.
I'd suggest you post your questions here: Family Cruising - CruiseMates Cruise Community and Forums
where you can get the "real" answers from parent's