Well, that's were we disagree, isn't it? We are not saved by water immersion.
I know your worldview has you predisposed to believe that salvation occurs at the instant one gives intellectual assent (belief) to the Gospel message. And if John 3:6, Acts 16:31, John 1:12, John 8:24, John 6:40, John 11:25, John 20:31, and many others were the only passages that speak of salvation, listing only belief as the prerequisite, then I would agree with you.
But there are also John 3:36, Acts 2:38, Rom 10:9-10, Acts 3:19, Acts 22:16, Matt 7:21-23, and Gal 3:27, which are also equally Scripture, and as Scripture have equal weight in the discussion of what is required to receive salvation. Not a single verse that says something leads to salvation can be excluded or we end up with a distorted picture of what is required, and we may miss the crucial step.
For instance, can you believe that Jesus is the Christ, and believe that He died for your sins, and still live a sinful lifestyle (theft, prostitution, adultery, homosexuality, deception, fraud, etc.) and still be saved? NO! Because repentance is required for salvation. You cannot be saved still planning to live a sinful life.
Also, if you have not publicly declared Jesus as your Lord, then it doesn't matter what you think, He has not claimed you as His child (Rom 10:9-10, Matt 10:32-33, Luke 12:8-10). That means that when the minister says, "heads bowed and eyes closed" so you won't be embarrassed by raising your hand to claim Jesus as you Lord, Jesus puts His head down and closes His eyes too, and doesn't claim the people too embarrassed to claim Him.
Also, Gal 3:27 says that all those who are baptized into Christ have put on Christ. If you have not been baptized into Him, then you are not clothed with Him. You are still in sin and unsaved. John 3:36 makes equal belief and obedience. It says, if you believe you have life, but if you don't obey you will not see life and the wrath of God remains on you. Matt 28:10 commands the teacher to baptize the learner, and Acts 2:38 commands the learner to submit to baptism. This action is taken by man, and 1 Pet 3:21 and Acts 8:35-36 place that action in water. And Mark 16:16 says that this action results in salvation.