There are certainly nodes in Kelp'thar Forest as well, so if the route through the two other sub-zones seems barren, you can try your luck with a swim around Kelp'thar to supplement your farming. The red line on the diagram shows a path that covers many of the nodes in Shimmering Expanse and Abyssal Depths. Unlike other zones, the ore nodes don't necessarily occur in the hilliest areas and are more dispersed here. Of special note to players who have both mining and herbalism: the optimal route for each profession is nearly identical. You will find many Azshara's Veil and Stormvine to go with the Obsidium ore.
Vashj'ir is unique because the majority of it is underwater. Unless you are a druid and have seal form, you need to do the first half-dozen quests (or thereabouts) in the zone in order to get your Subdued Abyssal Seahorse mount, which helps immensely for farming
It seems that Vashj'ir is one of the least farmed zones, so it is possible to find a lot of nodes up and ready for farming. This might also cause prices for the two herbs that are unique to the zone to remain steadily high for a longer period of time. As time passes, there is a good chance that Obsidium Ore will follow the same trend that its Wrath of the Lich King counterpart Cobalt Ore followed: That it will attain a higher sustained value than Elementium Ore because it is farmed significantly less frequently. The small barriers to entry to the Vashj'ir zone may keep certain dedicated farmers away, though it is difficult to predict where the equilibrium will eventually be reached. Each realm has its own foibles, and also much will depend on the production side of the professions to see if Obsidium or Elementium is more profitable.
Mount Hyjal Mining Route
Vashj'ir Mining Route
Deepholm Mining Route
Uldum Mining Route
Twilight Highlands Mining Route
Tol Barad Mining Route
Mount Hyjal Herbalism Route
Vashj'ir Azshara's Veil and Stormvine Route
Deepholm Heartblossom Route
Uldum Whiptail Route
Twilight Highlands Twilight Jasmine Route