It looked to me like Din fell straight down into the water. And Bo-Katan's explanation that the Imperial bombing might have caused a collapse seemed plausible in light of all of the damage on the surface. I was struck by how she asked Din if he had seen anything down there, and that he said he had not. As for the water itself? The Armorer pouring the sample into her basin, and how the water reacted seemed to be proof enough to her that they had been in the waters. I suspect that the colors that develop when the water is poured might be signs of Beskar ore that has leached into the water. Besides that, the Armorer seems to have her own supply of the water, since she used it in the first episode of the season, and she has also used it before. Has she been back to Mandalore and not said so?
Bo-Katan's arrival with Din Djarin to the covert seemed to escalate the tensions among them. Paz Vizsla seemed on edge about her presence. And I suspect that past differences between the various Houses and Clans of Mandalore might be the cause of future conflicts. There have been theories that the Armorer is Rook Kast, who was part of the Death Watch sect that supported Maul during the Clone Wars. Her armor resembles the character seen in the Siege of Mandalore, and the horns on her helmet might lead you to think that she was part of Maul's faction. Kast was aligned with House Vizsla at that time. And then to see Bo-Katan welcomed by the Armorer into the tribe after her immersion in the waters? I'm really curious to see where this goes.
I think the big question from this episode is about the Imperial Remnant. Moff Gideon had TIE Fighters. We saw several of those in his Remnant. But the TIE Interceptors and Bombers? That was a serious escalation from what we had seen before. And the storyline with Dr. Pershing and Gideon's former comms officer, Elia Kane (she finally has a name!), seems to be leading towards something developing in the way of another Imperial Remnant faction.