The seals we see as the Rhino goes through the ice. We're told the ice is 10 foot thick (and presumably has been there for some time) so how were these able to breathe?
Not having seen the episode yet (sigh... back in Canada now...), I do think, however, that I'm able to provide an answer concerning this particular problem.
There usually is a small pocket of air between ice and unfrozen water of lake or river. Survival skills classes will teach you that you can breathe in this pocket of air if you fall through a hole and get trapped under the ice (that, if you can withstand the cold! Which, admitedly, you will not do for long...) This is also what permits animals such as seals or polar bears to swim in cold waters, under the ice, from one hole ice to the other - they instinctively know they would find air there.
So, most probably in the case that interests us, the animators did their homework before including seals in this scene...
That... or this was all a huge mistake and I just provided them with a good save!