Thanks for your question.
Actually, there are almost no reasons why a person cannot be submerged in water. I have witnessed the baptism of a very frail terminally-ill cancer patient. She was baptized in her own bathtub at home and died about a month later. You cannot decide the form of baptism on the basis of hypothetical exceptions. God is just and is quite capable of making decisions about salvation for those in exceptional circumstances. That is up to Him. We, on the other hand, are constrained by what the Bible actually teaches. The Bible clearly teaches that baptism is absolutely necessary for salvation.
Here is some information on the website which will help you with this question. Please feel free to ask any follow-up questions that occur to you as you read this material:
I hope you have found this helpful.