Questions & Answers about Сегодня жарко, и дети плавают в море.
Why is there no word for "it" or "is" in Сегодня жарко?
In Russian, general states of the environment or weather use an impersonal construction. You just use the adverb жарко (hot). You don't need a dummy subject like "it" or a present tense "to be" verb like "is".
How would I say "The children are hot" using the grammar from this lesson?
You would put the experiencer in the Dative case. "The children are hot" is Детям жарко (literally "To the children it is hot"). In the sentence above, жарко describes the general weather, but adding a Dative noun or pronoun makes it about someone's personal physical state.
Why use плавают instead of плывут if they "are swimming" right now?
Even though it is happening right now, the children are swimming around for fun, not heading toward a specific destination. For general, multidirectional swimming, you use плавать (here, плавают). If they were swimming across a river in one straight direction, you would use плыть (плывут).
Why didn't the ending of море change when used with в to say "in the sea"?
It actually did change into the Prepositional case to show location, but the ending happens to look identical. For neuter nouns ending in -е like море (sea), the Prepositional case ending is also -е, so the word looks exactly the same as its dictionary form.