Back to your question about transfers or cab: I wasn't clear on whether you were asking aoubt L.A. or Miami . . . or both.
Penny gave you a good answer for Miami.
If you decide on the ship transfer it would probably be easier to use it in both ports. But I wanted to let you know that in L.A. a third choice would be either Super Shuttle or Prime Time Shuttle. The fare from LAX to the terminal in San Pedro is $17 per person. Not only do you save money, but you can reserve and pay in advance; and you don't have to wait for a bus to fill up. Don't even consider a cab in L.A.--they are outrageous.