>> I am worried we may not made it on time for our cruise next day in Miami.<<
uop, you really are cutting it fine by planning to arrive just before a cruise departure. You should have allowed contingency for that, especially not flying in on the last flights the day before. It's not just the potential misconnect but the possibility that last IAH-MIA of the day could be canceled.
But, all may not be lost ....
>> My departure is next April, 2014.<<
That's a long way off and hence there's always the possibility of a schedule change for either flight. Watch your reservations frequently, and if the flights are rescheduled meaning United offers you alternatives, use that to negotiate (politely) for earlier flights.
If that doesn't happen within say 2 months of departure consider paying the change fee and fare difference and change your flights. ..,.
Or .... actually make that And ...... make sure you get a good travel insurance policy that covers you for "missed onward travel", whereby if you miss the cruise departure you're covered for an airfare to the cruise's next port of call to pick it up there.