QUOTE (Street Magic @ p. 173)
Note that some elemental walls (fire, smoke, etc.) are not solid and will not block attacks, though they may inflict Visibility modifiers as determined by the gamemaster. Solid walls (earth, ice, etc.) have an Armor and Structure rating equal to the caster’s Spellcasting hits.
I assume you are asking about water because it is conspicuously not listed in either set of examples, and because it isn't technically "solid". But if you look at the elements that are NOT solid, and determine what about them makes them not count as solid, it seems pretty simple to figure out where water falls.
Fire is superheated gas: such gas offers almost no resistance against bullets or projectiles, and is pretty much no different than firing through normal atmosphere. Likewise for smoke, diffuse particulates suspended in the air aren't going to significantly impede ballistic or projectile flight. Other magical elements I would group in this category are Sound, Light, Electricity, and Blast, as they all lack physical components and are instead comprised of energy moving through the medium of the atmosphere.
Solids, liquids, and solids acting as liquids (such as sand), shoulld all count as Solid Walls.
Gases, gas-suspended solids or liquids (such as smoke or mist), and elements comprised of energy (such as Sound, Blast, Light, and Electricity) should all count as Non-Solid Walls.
So my final breakdown is as follows.
Solid - Earth, Ice, Water, Sand, Metal, Acid
Not Solid - Fire, Smoke, Blast, Light, Sound, Electricity
~Umi
Addendum - For the sake of completeness, hypothetical elements comprised of liquid or solid suspended gas (in essence Foams) would be difficult to class. Some foams are remarkably solid, others are remarkably not. It depends largely on the material the gas is suspended in. Light and frothy sea foam, or foam off the top of a beverage, isn't going to stop anything. Something thicker like industrial or structural foam would of course be on par with a wall of water or sand. It's a question of ratios, ultimately.