I'm no expert, but I searched for it and this is what I think I've discovered
The image you captured appears to be 'Hypselodoris olgae'. The image that you linked in post #3 is not the same as the critter you photographed - that link does display 'Hypselodoris Marci'.
For more detail...
If you go to the link below, then scroll to the post dated June 25, 2007, you'll see a comment and photo submission. The critter - in this particular submission -appears to be identical to the one you photographed. Colors on rhinophores, even the color/pattern variation mid body. Bill Rudman responds to the poster explaining what this critter is.
In seaslugforum, some where initially calling it 'Hypselodoris Marci'. Bill Rudman then determined it was not 'Hypselodoris Marci' and temporarily called it 'Hypselodoris sp.5'. Subsequently, it was named 'Hypselodoris olgae'.
Here's the
link (scroll down to June25, 2007 submission):
The Sea Slug Forum - Hypselodoris olgae
If you read the entire page, you'll find links to 'Hypselodoris Marci' on seaslugforum - which doesn't look like the nudi you photographed in Cozumel, but which do look like the nudi linked in your post #3.
I went over to your flickr page and clicked around - nice work!
