Hi Spitfire,
Please visit our local public transportation website. There is a bus route map, links to updates on bus routes and a number you can call for specific route advice 305-891-3131 (that's what I recommend the most -- talk to a person.) Website here:
Transit
Edwin is right about the cronyism, but that doesn't affect day to day business on the metrorail. It's a great way of getting around but in your case won't help you get straight to the beach. If you lived in South Miami, for example, you could use metrorail plus bus. Or if you lived in Brickell, you could take the bus or a metromover/bus combo to the beach. (Metromover is a free electric rail that goes between Brickell and downtown.)
The other thing to keep in mind is when you plan on commuting. I believe some bus routes are slower at night. You could spend a lot of time waiting and that's going to be awful if you get stuck in the rain.
Have you looked into rent prices and sharing an apt (if you come here with people?) You may not need to live off south beach. Also consider other areas of Miami Beach that are straight off Collins Avenue. That's a north-south bus route. You could live in mid-beach and work in south beach. Rents tend to be cheaper north of south beach.
By the way, I realize it's a personal preference, but the idea of getting around in a motorcycle in Miami gives me chills. Our drivers are crazy and inconsiderate and it's so dangerous. I would never recommend a motorcycle/scooter for commuting unless you were going really local, like say, sticking around the beaches or just tooling around particular neighborhoods and even then I would still hesitate.