Submariner and Sea-Dweller: Both are diving watches; the Sub is water-resistant to 300m, the SD to 1000m (and has helium escape valve in the side of its case). The Sub has the magnifying cyclops on the crystal (although there is a no-date version of the Sub which, naturally, does not have the cyclops), the SD does not. The SD case is thicker than the Sub's. Both use the 3135 calibre. (The LV Sub has a green bezel and Maxi-Dial)
The GMT-II is a multiple timezone watch, with the ability to track 3 different timezones using the normal hour hand, the 24-hour arrow hand, and the bezel. It has recently been updated to have a more substantial case, larger markers on the Maxi-dial, a ceramic bezel, and a larger Triplock crown (like the Sub/SD) although is rated to 100m water-resistance. This runs the 3186 calibre. The older version of the GMT-II had a thinner case, smaller dial markers and smaller Twinlock crown.
These are the main obvious differences that are immediately obvious (with respect to the steel versions of these watches). However, there are many other differences between the watches which are known to the enthusiast.
Have a look at our Rolex Picture Gallery http://newturfers.com/bin/mwf/board_show.pl?bid=29