What real-life animal(s) would be realistic to use in my story?

I'm confused with your condition of "realistic" in this case. I'm sure that there's no such animal where this realistically can happen. To the best of my understanding, no animals have direct connections between reproductive and digestive systems.

You might as well throw out that condition and go all surreal/fantasy scenario.

A whale ?

The man is swallowed head first by a whale while swimming

He then uses his divers knife to blindly cut his way out, by chance exiting via the whales genitals

Would he be have time to exit her genitals before she died of her cuts?

I don't know, it's never crossed my mind how far a whales stomach is from its genitals

A sperm whale is the only one that could possibly swallow a human whole.

All other whales have a throat that is way too small.

So say you were swallowed whole by a sperm whale. There would be no breathable air in the stomach so you would likely suffocate within a few minutes. And to add to that sperm whales do have teeth that is used to catch their prey so if you were able to get beyond the teeth, then their stomach walls are used to grind up their food so the human would likely be ground up; I would imagine it would be tough to cut through the thick muscular stomach and if the human was able to live through that, then they would go to the other stomach chamber where there is gastric juices.

I'd say no it is not possible.

Some guy did try to get eaten alive by a large anaconda, while wearing a protective suit. It didn't go well as when the snake attacked it was breaking his arm as these snakes suffocate their prey to death before swallowing.

I think wolves can swallow people whole, or it might just be grandmas.

Land Of The Lost - Grumpy Eats Marshall & Escape Route

What? Eating boogers in the back of a classroom isn't enough, you need to come up with this codswallow to annoy adults with? ******* incels.

I'd say improbable, but not impossible

His scuba diver mini oxygen tank gives him 10 minutes

Divers knives tend to be like steak knives, very sharp

His dry suit would buy him time before it dissolved

The adrenalin rush would concentrate his mind and give him strength


The mind boggles

There are millions of people who believe that a man by the name of Jonah was swallowed by a whale and remained alive inside that big fish for three days and three nights, so that might be a good animal to begin with.

In 1933 a sulfur bottom whale, also known as a Blue Whale was caught off the coast of cape cod it was 100 ft long and had a mouth over 10 foot wide. A man who was unfortunate enough to be swallowed could take refuge in any one of the whale's stomach chambers, or the large cranial cavities (extensions of the nasal sinus) which measure 7 feet high, 7ft wide and 14 ft long. More than big enough for a man to hide safely inside and in which, he might receive an ample supply of oxygen and be protected from the corrosive stomach acids.

Baby blue whales are 22-24 feet long and weigh around 6,000 pounds when they are born, so a human could easily escape through a female Blue Whale's genitalia.

I hope that is of some help to you.