Yes, we used to go on a Saturday morning and return Sunday but then the boat is just an hour from our house. Most folks might go down on a Thursday or Friday, some even staying the week. I see they have changed the schedule, leaving both Miami and Key West later.
Be sure to check the website for the most current time and rate info! Just pulled the following off their site. It is subject to change.
http://www.seakeywestexpress.com/
Fares from Miami to Key West
Miami Fares inclusive of all charges
Adults
$102.00 per person round trip
One way $51.00
Juniors
Juniors up to 12 years
$70.00 round Trip
One way $51.00
Seniors - 62+
Seniors - $83.00 round trip
One way $51.00
College Students
with current ID
$83.00 round trip
All one way fares between Miami and Key West are $51.00
Depart Miami 10:00am Arrive Key West 2:00pm Depart Key West 7:00pm Arrive Miami 11:00pm