The question of whether baptism is essential for salvation is really rather easy to answer. We are saved by grace, through faith, not of works. Baptism is a work, and thus it is not a part of salvation, but rather an indicator thereof. It is a pledge of good conscience before God (1 Peter 3:21), not a means of salvation.
Nevertheless, the debate rages on. One way I like to defend the fact that it is not necessary for salvation is to bring up a hypothetical scenario in which a number of new converts are due to be baptised. They enter the water to get baptised one by one, however the service is interrupted. When only half of the new converts are baptised, a terror attack happens, and every new convert is killed. Question: How many of them made it to Heaven?
The correct answer is "as many as had faith". If, for example, we had 10 new converts, with only 5 managing to be baptised, we could say that all 10 went to Heaven if they all had faith. But if you believe baptism is essential for salvation, you can only answer a maximum of 5. The other 5, even if they had the exact same faith as the 5 who were baptised, went to Hell. But if you believe this, you do not believe, as Ephesians 2:8-10 teaches, that salvation is by grace through faith, and not of works. You believe salvation is by baptism through faith, not of grace (Romans 11:6).
As it turns out, however, we don't even need the hypothetical, because we have an example in Scripture of someone who had faith, and was guaranteed salvation, yet died before he was baptised. In Luke 23:39-43, we read that one of the criminals who was being crucified beside Jesus converted there. Obviously, being crucified, he had nothing to offer but faith. Jesus told him he would be in Paradise with Him that very day! No baptism, yet he was saved.
This is where people start making excuses. Maybe the criminal was baptised before he was nailed to the cross? Not only is this ad hoc, it doesn't fit very well with the evidence in the account. It seems the criminal was very recently converted. Maybe he was saved under the Old Covenant? Not only is it daft to suggest that the Old Covenant was significantly lighter than the New, but we know for a fact that the New Covenant was ushered in the moment Jesus died (Luke 22:20; John 19:30; Hebrews 9:15-17), and He died first (John 19:32-33). The daftest excuse I've heard is that Jesus can make exceptions if He wants to. But tell me, is God arbitrary? Does He play favorites? Even if He does make exceptions to a rule I believe I have shown is contrary to what Scripture actually says, why would He make those exceptions? Answer: Faith! Of course faith! But this is the very criteria Scripture presents! Salvation by faith apart from baptism isn't the exception to the rule, it is the rule!
Ultimately, while baptism is a very important part of the Christian faith, to claim that it is required for salvation is antithetical to the Gospel message. Even those who claim to believe it prove that they actually don't whenever they come to even a hypothetical scenario wherein baptism, for one reason or another, is impossible. When presented with actual scenarios wherein the unbaptised are explicitly saved, such as the thief on the cross, they may make a number of excuses, but ultimately, it is impossible to get around the fact that, if their faith is true, the unbaptised are saved. It is simply unfathomable that, if their faith is indeed true, they would want to remain unbaptised.