I'm no expert but it looks like a large portion of them wander up and down Melgar all day long, back and forth, with a sort of glazed look in their eyes. A bunch makeit as far as Carlos n Charlies, or Senor Frog and kill the day off there until the ship blows it's horn, summoning them back. I've done several cruises with clients, and we did the very same thing on almost every island as our time there was limited and we couldn't always get an excursion in.
Seriously though, I assume the poster on this board dives, and again, I'm no expert but our experience is that we're out diving for the majority of time the masses are out in numbers. In the afternoon we shower, chill out, and by the time we're ready to head out for a late lunch we go a block or even two off Melgar we're out of the crossfire. You'll do better hanging out off Melgar anyway, with or without the cruise ships since it seems a lot of what's there is "propped up" specifically for that crowd. The few places we like on Melgar like Rock 'n Java, don't seem to attract the "cruisers" as much as the more familiar "chains". Go figure, you travel all the way to Cozumel and then eat at Subway.
A lot depends on your perspective, the cruisers do fill the streets, but it also makes for a festive atmosphere and it's always fairly serene by sundown.
Have fun and know you're gonna love it there!