Many large aquatic animal such as whale sharks and humpback whales are filter feeders, mostly eating things like plankton and krill.
But what if a large animal such as a seal or human accidentally gets sucked up? Does the animal lack the ability to digest such a large meal? Do they just swallow it anyway? Do they spit it out?
I once saw a nature show (sorry no cite) that showed a group of Humpback Whales using bubble nets to “round up” herring (I think) in Alaska. Herring are generally a small sized fish, but can easily reach 6 inches in length, so they don’t just feed on plankton and krill.
As far as what would happen in the event that something as large as as a seal or human somehow got into their mouths, I believe their throats are large enough to swallow and then digest something that size, which is what would probably happen.
Certainly a Blue Whale’s throat would be large enough to swallow a human… not sure about a Whale Shark’s though.
Assuming you have an air tank and such, how long would it take a blue whale’s digestive juices to kill you? Minutes? Moments?
Not true. The Blue Whale’s throat is only about 10 inches in diameter at most (I’ve seen some cites down to about 4 inches).
Baleen whales feed by first taking in a mouthful of water containing food items, then pressing the tongue against the sheets of baleen to expel the water and strain out the food. If they take in an object too big to swallow, they can presumably sense it at this stage and eject it with the tongue.
I saw on a nature documentary a seagull that was floating on the ocean surface, picking at small fish. A baleen whale came up from below, mouth opened wide. The bird tried to fly away, but the mouth closed around it too quickly. Bird eaten by whale.
I imagine Colibri’s right about it sensing something too big and spitting the bird back out. At that point, though, the bird has most likely been crushed between the tongue and baleen and/or drowned.
Ok, it’s a bit of snark, but we’re totally ignoring the documentation in the Bible about a whale swallowing a human? 
(Sorry, I’ve been reading certain other threads)
This column by Cecil (which I helped research) may be of interest:
Have any real-life Jonahs been swallowed by whales and lived?
Creepy questions that will keep me up tonight imagining. Brrrrrr.