Baptism is when you get saved (1 Peter 3:21, Acts 2:38, Romans 6:4, Acts 22:16). There is nothing powerful or magical about the water, God does the work when you obey Him. So it can be any body of water that is enough to fully bury your old sinful self to be raised a new creature alive with the Holy Spirit (2 Corinthians 5:17)- Who marks us for salvation (Ephesians 4:30). Baptism is how we get into Christ, and those who are in Christ will be saved.
We consider it an emergency. If you drop to the ground having a heart attack, do you wait until another time to call an ambulance? Do you wait for an invitation? No, if someone realized they needed to be baptized in order to be saved, right then a christian should have a quick talk with you to make sure you understand what you are doing, that you no longer have the mind of a child, and that it is a lifetime commitment. Then you should be baptized immediately- even if it is in the middle of the night, even if it is in the middle of a sermon- someone should take you aside and and begin the process. Do not let anything hinder you if you are serious, because your spiritual state at the point of death is now sealed for all eternity.
I saw on the news once that an airplane crashed into a house, and those inside died instantly, and whatever their spiritual state was is now sealed for all eternity with no other chance of changing it. My brother in Christ also told me of a time that he was at church and the roof fell- killing many people instantly, he himself came close because the people in the pew behind him died. You never know, your days, hours, minutes, and even seconds are not in your control.
Once you come in contact with the life-saving blood through baptism, you keep in contact with it by taking the Lord's Supper in the assembly of the saints every first day of the week. It is unleavened bread (most view as a cracker) without yeast to make it a fluffy bread- because it symbolizes Christ's body without sin. And the fruit of the vine (grape juice) is pure to symbolize how Christ's blood was pure. Now when water flows through the grape seed it produces grape juice within the grape- therefore it is ok to use considerate and add water. But you cannot add anything else, and it cannot contain alcohol. I know this juice was called wine in the bible, and we call juice with alcohol wine today, but no matter what you call it, Christ's blood was pure and the symbol of His blood must also be pure. When Jesus performed His first recorded miracle of turning water into wine, He did not make it to have alcohol that people could get drunk on it. He knew how much they were drinking- which is why they ran out. Jesus did not support the sin of drunkenness. The drink He made was pure grape juice.
Jesus says, "I AM the Bread of life." And He says, "Whoever eats My flesh and drinks My blood has eternal life, and I will raise him up at the last day." John 6:54 But you first have to be baptized into Christ before you start taking the Lord's Supper. And you have to examine yourself before you take it, to agree that you are walking in the light to the best of your ability and not still living in sin (There is a difference between visiting somewhere and living there). Because whoever takes of the Lord's Supper in an unworthy manner is guilty of sinning against the body and blood of Christ (woe!).
Now the church is the bride of Christ, and He only has one bride (body/church). So if you were baptized into a false religion, and realize this, and get into His true church, do you need to be re-baptized? Let me answer that question with a question. In Romans 7:1-4 it tells how we were married so-to-speak to Moses (spiritually speaking of course). And that he died and we are now married to Christ (so-to-speak). Now if you were getting married to someone new, would you say to them, "We don't have to get married cause I have already gone through the process of getting married"? They might say, "Yeh, but not to me!" Yes, Christ has only one body (church) and you have to be baptized into it. I personally have studied many religions and have found only one to match the one true church of the bible- the Church of Christ, but you need to study on your own and see the true church for yourself.