To get to Isla Mujeres you would need to take the 10am ferry to Playa del Carmen, arriving at 10:40 or so, then walk 3 blocks to the bus station, take a bus to Cancun (they run every 15 minutes) and arrive 1 hour 15 minutes later. Then a taxi to where you catch the ferry to Isla. I'm sorry, I don't have the schedule for that ferry. It can be done, I'm sure, but involves some planning and patience. Also, you have to gauge whether or not it will be worth the time spent in transport.
If you are still interested, let me know and I'll work on getting that ferry schedule for Isla to you...
Never mind, found it! There are two ferries. One ferry to Isla departs at 9:45am, 11:30am, 2:30pm and 5:15pm. Return trips at 1:30pm, 4:30 and 7pm. The other departs at 9:30am, 11am, 12:30pm, 1:15pm and2:30pm. Return trips at noon, 3:30pm and 5pm.
The ferry from Playa del Carmen back to Cozumel runs at (I'll limit to afternoon/evening times here) 3pm, 5pm, 6pm, 7pm, 9pm and 11pm.
Hope that helps!
Minigolf Sally
Cozumel Putting Course