You assumption is incorrect. When the ship returns to Ft. Lauderdale you MUST go through Immigration. It doesn't matter how you booked the cruise, this is U.S. law. Follow the instructions provided to you exactly. Read my previous post for more details.It depends on what the previous last port of call was. If it was a foreign port, then yes, you must clear immigration at Ft Lauderdale. If it was a US port, then no, you do not have to clear immigration again as you would have cleared at the previous port of call (assuming the port before that one was a non US port). We did two HI circles out of San Pedro. The first went straight to HI and then Ensenada on the way home to meet the foreign port rule. As San Pedro was our first port of call after a foreign port, we cleared customs and immigration when we disembarked. The second cruise first stopped at Ensenada and then went onto HI. We were required to clear immigration prior to docking in HI. On disembarking in San Pedro when we got home, we were only required to clear customs. Immigration was not required as we visited no foreign ports after leaving HI
We did B2b out of Fort Lauderdale last year and we did not have to leave the ship. We (and about 400 other odd passengers) did have to wait for over an hour in the Princess Theater as another b2b passenger didn't think the immigration instructions applied to them so they didn't bother to present themselves to immigration. They were only "caught" when they tried to use their cruise card at the IC.