We did the trip, Sanibel to Marathon as follows
Day 1. Sanibel to Marco Island where we went inside to the old ICW to Goodlands where we andchored at nearby Tripod Key for the night. 54 st. miles
2. Tripod Key out to the Gulf through Coon Key Pass to Little Shark River Where we spent the night at achor. 59 st. miles
3. Little Shark River to Marathon Marina. 59 st miles
Both anchorages were very nice, the "old ICW" has some shallow spots but we had no problem with our 4' draft.
We didn't do it but some folks make the trip up to Everglades City. Some love it there, others not at all. Maybe we'll try it next time.
LOTS of crab trap floats everywhere.
If you like take a look at the blog link below, the March 2010 index will show the entries for the trip and link to SPOT images of the anchorages we used.