Oceanside Marina is a nice place, and has the best Yamaha mechanic in the area. Oceanside is actually on Stock Island, one key in from Key West. Probably the best ramp on Key West is Garrison Bight, run by the city, but there is also a good marina/mechanic right there.
Both places charge for ramp use and parking. I'm also not too sure about either place allowing you to tie up and sleep on your boat, but Garrison does rent transient space and Oceanside's web site seems to indicate they do. Garrison is consideribly cheaper, marginaly less secure, but a lot closer to everything else Key West offers. There isn't really any significant difference in the transit times to the reef from either location, but Oceanside is closer.
http://oceansidemarina.com/ http://www.keywestcity.com/depts/por...on%20Bight.asp
I'll be around this weekend if you need any help with anything locally, though I can't help out too much with Fishing advise...
Make sure you have a local chart if you are going to do anything but head out the channel to the ocean, and Florida Saltwatre Fishing Licenses are required, and there are a handful of sanctuary areas that are off limits to fishing...and are well patroled.
A few links from a previous post of mine:
Make sure you have a Florida SW Fishing License (or you are exempt)
https://www1.wildlifelicense.com/fl/
And take a look at the regulations:
http://myfwc.com/fishingareas.html
And beware of the National Marine Sanctuary areas:
http://floridakeys.noaa.gov/regs/welcome.html
That should keep you out of trouble. Rumor has it the Fish and Wildlife guys may pay a little more attention to visitors on our water...so ensure you have everything in order. They all seem to be pretty decent folks, but Keys people are very sensitive about the resources.
And if you haven't done Keys boating before, there is some good info on not running aground here:
http://nps.gov/bisc/visit/groundings.htm
Be careful, Keys navigation is a bit more advanced than other areas. Overall its very safe around here, you just have to pay close attention to channels and such.