Let's look at the sentence: The lodge sits on the water, and beavers swim under to get inside.
This sentence is mostly good, but we can make it a little clearer and easier to understand.
First, the word "under" means beneath something, but here we want to say "under it" meaning under the lodge.
So, a better sentence would be:
The lodge sits on the water, and beavers swim under it to get inside.
We added "it" after "under" to show that the beavers swim under the lodge.
Great job looking carefully at the sentence!