In the Catholic Church, holy water is water blessed by the priest with solemn prayer, to beg God's blessing on those who use it and protection from the powers of darkness.
Throwing holy water on someone is an easy test to determine if they possessed. See Holy Water in the Face
While planning to exorcize the Airplane Demon, Dean brings holy water to find out who is possessed, but Sam, wanting to be subtle, tells him to speak the name of God in latin; "Christo", knowing that demons flinch when they hear it. Dean uses "Christo" on Amanda Walker, but it doesn't affect her, and with the EMF reader he soon realizes that the copilot was possessed. While Sam is attempting to exorcise the demon, Dean splashes the holy water on him, which sizzles his skin. The demon is later sent back to Hell after Sam finishes reciting the exorcism.
Bobby blesses the water in a sprinkler system. He uses the sprinklers to create a wall of holy water that Lilith's soldiers cannot penetrate. It is stated by Ruby that Lilith herself has some immunity to holy water.
Dean tortures Alastair with holy water.
The townspeople have a unique means of delivering holy water: a fire engine. They bless the holy water in bulk and spray it at demons with a fire hose.
Though he is in theory retired, Dean takes the precaution of keeping a mason jar of holy water under his bed.
Sam and Dean splash holy water on Linda Tran to make sure she isn't possessed.
Dean pours holy water on Arthur Swenson to test if he is possessed.
Linda Tran squirts Kevin and Delta Mendota with holy water to make sure they aren't possessed. She also shoots a demon with holy water from a water gun and captures him.
Sam and Dean splash Henry Winchester's face with holy water.
Kevin believes he is using holy water against Sam and Dean, who turn out to be demon possessed fakes.
Sam lists holy water among the things they have brought to protect Sarah Blake from Crowley.
In the Bunker, handcuffed to a chair inside a Devil's trap, Dean lashes out at Sam, baring his teeth. However, Sam is ready with the holy water and splashes him in the face. Later on, Sam throws more holy water into Dean's face to see if the Demon cure has worked or not.
Cole has captured a demon in a devil's trap and begins torturing it for information on Dean with holy water. Later when Cole tracks Dean down, he throws holy water into Dean's face, which has no effect other than annoying Dean.
In order for Dean to figure out which, if any of the wrestlers are possessed by a demon, he serves up shots of tequila laced with holy water. To no avail, the wrestlers did not appear to be possessed and left Dean drunk and passed out in the bar.
A hunter in Apocalypse World throws holy water on to Mary to test if she is a demon.
Exorcism of Water
Now, acquae sounds more like oque but it only makes sense if it's acquae, lit. 'of water' - however, John's pronunciation sounds a little clearer than Sam's. I don't know if this is due to the different emotional situation in which the two have to pronounce the Latin (Sam's in fact more stressful than John, in both instances) or simply to Jared Padalecki and Jeffrey Dean Morgan's accents.
Quick and easy, I do believe that again this is a quick ritual put together by the writers. however, given the previous information found, the fact that it's John (or Sam in 1.04 Phantom Traveler and 1.22 Devil's Trap) pronouncing the exorcism and not a priest doesn't make the ritual ineffective.
To be strictly theological, I suspect that it's not an exorcism John should have performed, but a blessing. The exorcism implies that there's a demon in that water, whereas a blessing would have turned the water into holy water. So, technically, its a mistake.
The following is the entire Exorcism of Water that was in official use prior to the reforms of Vatican II:
"O water, creature of God, I exorcise you in the name of God the Father almighty, and in the name of Jesus Christ His Son, our Lord, and in the power of the Holy Spirit. I exorcise you so that you may put to flight all the power of the Enemy, and be able to root out and supplant that Enemy with his apostate angels: through the power of our Lord Jesus Christ, Who will come to judge the living and the dead and the world by lire. Amen. Let us pray.
O God, Who for the salvation of mankind has built Thy greatest mysteries on this substance, water, in Thy kindness hear our prayers and pour down the power of Thy blessing + into this element, made ready for many kinds of purification. May this, Thy creature, become an agent of divine grace in the service of Thy mysteries, to drive away evil spirits and dispel sickness, so that every- thing in the homes and other buildings of the faithful that is sprinkled with this water may be rid of all uncleanness and freed from every harm. Let no breath of infection, no disease-bearing air, remain in these places. May the wiles of the lurking Enemy prove of no avail. Let whatever might menace the safety and peace of those who live here be put to flight by the sprinkling of this water, so that the healthfulness, obtained by calling upon Thy holy name, may be made secure against all attack. Through our Lord Jesus Christ, Thy Son, Who lives and reigns with Thee in the unity of the Holy Spirit, God, for ever and ever. Amen."
Holy Water in Lore
According to Wikipedia the holy water blessing ritual is:
``Exorcizo te, creatura aquæ, in nomine Dei Patris omnipotentis, et in nomine Jesu Christi, Filii ejus Domini nostri, et in virtute Spiritus Sancti: ut fias aqua exorcizata ad effugandam omnem potestatem inimici, et ipsum inimicum eradicare et explantare valeas cum angelis suis apostaticis, per virtutem ejusdem Domini nostri Jesu Christ: qui venturus est judicare vivos et mortuos et sæculum per ignem.
Deus, qui ad salutem humani generis maxima quæque sacramenta in aquarum substantia condidisti: adesto propitius invocationibus nostris, et elemento huic, multimodis purificationibus præparato, virtutem tuæ benedictionis infunde; ut creatura tua, mysteriis tuis serviens, ad abigendos dæmones morbosque pellendos divinæ gratiæ sumat effectum; ut quidquid in domibus vel in locis fidelium hæc unda resperserit careat omni immunditia, liberetur a noxa. Non illic resideat spiritus pestilens, non aura corrumpens: discedant omnes insidiæ latentis inimici; et si quid est quod aut incolumitati habitantium invidet aut quieti, aspersione hujus aquæ effugiat: ut salubritas, per invocationem sancti tui nominis expetita, ab omnibus sit impugnationibus defensa. Per Dominum, amen.´´