Let's look at the sentence: "The lodge sits on the water, and beavers dive down to get inside."
This sentence is mostly correct, but we can make it clearer and a little smoother for your age.
First, "The lodge sits on the water" is a good start, but beavers' lodges usually sit in the water, not on. We can say, "The lodge sits in the water."
Next, "beavers dive down to get inside" is okay, but since the lodge is in the water, we can say, "the beavers dive underwater to get inside." This helps explain they go beneath the water surface.
So, a better sentence could be: "The lodge sits in the water, and the beavers dive underwater to get inside."
Remember, adding "the" before "beavers" helps make the sentence clearer because we're talking about specific beavers near the lodge.