Then it is what I explained originally. Do this, going to break it down in simple steps.
Summon the Water Elemental > Open Spellbook > Click "Pet" Tab > Find "Freeze" Spell > Drag like any regular skill down to the Pet Bar and place in empty square.
Also, if you hate the Pet Bar, you can add the skill to your main action bars using a macro. I do this for a few skills that often require a bit more finesse, like Freeze. Here is a demo macro that should work.
#showtooltip Summon Water Elemental
/cast [target=pet,dead] Summon Water Elemental; [nopet] Summon Water Elemental
/cast [pet] Freeze
This makes it so the macro button can replace the Summon Water Elemental button. When you have no pet, or the pet is killed, it will summon a new water elemental. If you do have a pet, it will skip line 1 and instead make the pet cast "Freeze" when clicked. This is a great way to also prevent yourself from accidentally putting your water elemental spell on cooldown by summoning a second time when you already had one active. It won't let you summon unless the elemental is not there.
Also, for ease of use I usually recommend making a /petattack macro attached to one of your most used spells. The Water Elemental AI while in defensive will only start casting on an enemy after either the enemy damaged you, or you damage an enemy. This means to be most efficient you need to click the pet attack button on the pet bar before you start casting yourself. However, if you macro in the /petattack option into a macro with your favorite spell (like in this case probably Frostbolt) then the minute you start casting the pet will automatically attack the target you started casting on. If you switch targets, he will also switch targets with you.
