As of now in The Mandalorian Season 3, Bo Katan confided in The Armorer about having seen a Mythosaur in the living waters. She never told Din about it, and even The Armorer has kept the information a secret from him. There has to be a reason why Din doesn’t know what Bo saw in the living waters, and why he didn’t see it himself.
Before going into the water, it’s Din who recited the Creed, and then he fell in and sunk to the bottom. It’s possible that hearing Din speak the words of the Creed might’ve awakened the Mythosaur, or the fact that he fell into the water stirred the creature.
It could very well be that Bo Katan will tell Din about it in the upcoming episodes, but as of now it seems unlikely. This information doesn’t seem like something Din is meant to know, at least not until the right time. Back in Chapter 1 of the series when Din tried to ride a blurrg and failed, Kuill reminded him that his Mandalorian ancestors rode Mythosaurs, so riding a blurrg shouldn’t be a big deal for him. That was enough motivation for him to tame the blurrg and ride it. Could this scene have been foreshadowing events to come in Season 3? Could Din be the one to ride the mythosaur as well, or was that line just a coincidence?
Hopefully, The Mandalorian Season 3 will answer these questions, but for now, keeping the existence of a living Mythosaur from Din seems very deliberate by the writers, and might play a key role in the events that will unfold in the remaining chapters of Season 3. It might be Bo Katan or someone else all together who rides it, or the writers might pull a shocker and just have someone kill the poor creature before anyone gets the opportunity to, but for now, we can hope that the writers will give us a chance to see a Mandalorian riding a creature everyone believed to nothing but a children’s story.