Cozumel is fun, If cost is an issue I would aviod doing anything that is booked through the crusieline, as there will be a premium paid. It has been many years, but last time I was there, we hopped in a bad, and told the driver to take us to the marina, from there we negotiated someoene to take us out fishing, the cost was about 1/2 of what it would have cost to book a fishing trip from the cruise line.
Also make sure you actually go to cozumel, and not the little villiage built near the cruise line's ship dock.
With some more free time you could catch a boat to cancun which I believe is the next island over...