"I wish it was that simple but it is anything but. Harriet was actually designed specifically for these two passes and after it completes this final pass it will be essentially unusable. Perhaps even more important is that each boring machine is specifically designed for the type of soil, rock, etc. that it is meant to cut through. The soil located in Biscayne Bay is almost certainly very different then the one located on Miami's mainland which I'll explain next. If you look at a diagram of the port tunnel it actually cuts deep down below the ocean into the earths crust. The bay area is formed in a more typical way just like New York or Boston. Miami Beach may actually be formed this way as well but I'm not sure. It's worth checking out however.
Miami, however, is very different and unique. Right below the surface is the Biscayne Aquifer. This aquifer is what makes construction underground so difficult and expensive. the aquifer is a large freshwater body right below the ground, The ground is extremely pouris and the water basically is all in that porous ground. This is why you hit water so quickly and also why many buildings have to be built on piles in Miami. Hitting water instantly when trying to dig a tunnel makes it very hard because you are constantly having to pump out water while your work and also causes a ton of issues with structure and also future flooding issues among other things. heres a link with a diagram of the layers of soil (and water) below miami.
A tunnel with a park above I think would be the best option, but sadly it would also be amazingly expensive. Most likely several billion dollars :/ Hope my explanation was helpful."-MiamiArchi [FDOT Wants To Trash The I-395 Flying Land Bridge For This]