Hi deputy (aka: cjr),
Cozumel is a great place to snorkel. You might want to check out the Cozumel section of Port Reviews (our sister site). Tom has done extensive research and has provided a rather in-depth special report on the beaches of Cozumel.
Here's the link:
As for seasickness, you will find the cruise ship experience totally different than small boat cruising on the lake. Having spent numerous summers on boats in Lake Erie, I can assure you they are nothing alike. Whereas you feel every wave and ripple on small boats, unless you hit rough water, it is possible that you will feel very little (if any) movement at all.