Jonah 1:17 Now the Lord had prepared a great fish to swallow Jonah. And Jonah was in the belly of the fish three days and three nights.

There are people who ask the question Can a person survive being swallowed by a whale like Jonah who was in the whale for 3 days and 3 nights?

Yes they can. We have testimonies of those who have. James Bartley (1870–1909) in February, 1891, some two hundred miles east of the Falkland Islands in the South Atlantic was swallowed by a sperm whale and was in the Sperm whale for 36 hours and survive. He was found still living almost two days later in the stomach of the whale, which was dead from harpooning.

Deacon Chronicle In April 5 , 2016 states of Luigi Marquez a Spanish fisherman was in a whale for 3 days before being "flushed out" alive.

“It is the most frightening thing I have ever lived. Everything was pitch black and I was shivering cold. The only thing that kept me alive were the raw fish I ate and the light from my waterproof watch, that is how I kept in touch with time. And the smell, I will never forget that horrible stench of putrid decomposition. I had to wash for three days before the odor went away,” Luigi Marquez told reporters.

Other testimony of one who was in a whale is that of a Lobster diver named Michael Packàrd who on June 11, 2021 the ocean off the coast of Cape Cod was in the mouth of a Humpback Whale for 30 or 40 seconds before he was spit out.

A similar incident occurred off the coast of Port Elizabeth, South Africa, in 2019 when a Bryde’s whale scooped up a Rainer Schimpf a marine conservationist while documenting sardine run of mass migration. He was in the whale only 2 seconds before the whale let him free.

Also we have to look at the way the Hebrews saw day and night besides from evening to evening to them any part of the day would count as a would count as a whole day and the same in reference to the night .

JewishEncyclopedia.com DAY (Hebrew, "yom"):
By: Emil G. Hirsch, Michael Friedländer ("The term "day" is used also to denote a period of twenty-four hours (Ex. xxi. 21). ...circumcision takes place on the eighth day, even though of the first day only a few minutes remained after the birth of the child, these being counted as one day. ")

For example this why we see Jesus died on Friday but rose on Sunday which wasn't exactly 3 days and 3 nights of 24 hours.

Matthew 12:40
For as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth.

Matthew 16:21
From that time Jesus began to show to His disciples that He must go to Jerusalem, and suffer many things from the elders and chief priests and scribes, and be killed, and be raised the third day.

So Some see Matthew 12:40 & Matthew 16:21 as a contradiction which wasn't when we look at the following.

But let's remember it became dark at 12 pm till 3pm which counts a day & night

Matthew 27:45 Now from the sixth hour until the ninth hour there was darkness over all the land.

Then part of the rest of day of Friday when became day again, and whole night , whole day and night on Saturday. Resurrecting before the rising of the Sun on Sunday.

John 20:1
Now on the first day of the week Mary Magdalene went to the tomb early, while it was still dark, and saw that the stone had been taken away from the tomb.

So the same was with Jonah he could been in the Whale or sea monster for a part of the day or night maybe even less then 72 hours.

Also we have to notice that the scripture says the Lord had prepared a fish. This is the God that created heaven and earth in 6 days to him nothing is impossible.

Matthew 19:26 But Jesus looked at them and said to them, “With men this is impossible, but with God all things are possible.”

Luke 1:37 For with God nothing will be impossible.”

He could of easily made a special fish who had oxygen inside so Jonah could survive. Among those that could of swallowed Jonah whole could of been the following Great White Shark, a whale Shark, Grouper, sperm whale and there are others.

Now a other theory to take for consideration what if Jonah was actually dead for a time and revived in the whale before being vomited out. This theory comes from reading Jonah owns words " Out of the Belly of Sheol, The earth with its bars closed behind me forever; Yet You have brought up my life from the pit, O Lord, my God.“When my soul fainted within me"

Jonah 2:2,6-7
[2]And he said: “I cried out to the Lord because of my affliction, And He answered me. “Out of the belly of Sheol I cried, And You heard my voice.
[6-7]I went down to the moorings of the mountains; The earth with its bars closed behind me forever; Yet You have brought up my life from the pit, O Lord, my God.
“When my soul fainted within me, I remembered the Lord; And my prayer went up to You, Into Your holy temple.

Sheol the place of the dead after death is referred from Jonah's own mouth and the language of death is mentioned by him being raise from the pit , most likely speaking of death. Which makes him a perfect type of Christ in the Old Testament.