Grand Beach Hotel
I just posted this response to someone else on the Cruise Critic website but I thought it might also help you.
We are also going on a pre-cruise vacation to Miami and we booked the Grand Beach Hotel through SkyAuction.com at a rate that was about 60% less than their published rates.
It seems like a good hotel for families but there are also other listings for hotels between Miami and Fort Lauderdale on there.
Basically you bid and, if you win the auction, you give them your travel dates. If the dates aren't available then you aren't on the hook for the $$$. I've used them before and it's been a great service.