Lake Sylvia is quite nice we anchored there twice over the last couple of months. Once on the way to the keys and once on the way back. We draw five feet and I don't think we ever even touched the bottom. As was said though hug the wall on the way in/out. On our way back we were the only boat there until another showed up later in the evening.
The Las Olas marina has slips pretty cheap as well. if you are a boatus member you can get them for around $18/day for a 30' boat. Electric is on top of that with a $10 hookup charge.
The marina told us that the pumpouts they have are free. You can pull up and service yourself. We did not need it but I doubt they were lying.