"I will swim today."
There are two different types of sayings for kanji. On'yomi and kun'yomi. On'yomi are the "imported" Chinese readings and the kun'yomi are native japanese readings. Ima AND hi are the kun'yomi readings. However, each means now and day. So it means today. On'yomi readings are usually used when there are two or more per "word". So since there are two kanji together to make now and day meaning today and therefore pronounced kyō.
Someone please correct me if I'm wrong.
It is a godan verb and does not conjugate as such. The sentence is literally translated "I am swimming today" the intent of will is already implied because it is conjugated. 泳ぐ is the base form. Japanese does not have a future tense, so "will" is not really used. There are volitional forms, but they're suggestions more often than not, despite not being questions.
If you meant to ask "Why not wo (を) instead of wa (は)?", the answer is that を marks the object and there is no object in this sentence; you do not "swim something" the way you "read something" or "see something". "Today"/今日 is instead an adverbial (of time), and thus does not take を. It can take the topic marker は, if it is what is being talked about: "What wll you do today?" "I will swim today." If it is not the topic (the thing being talked about) it does not take は, and will then be completely without particle.
Japanese present tense and future tense is the same. It's kind of like how in english if you say "I bike" its more like you habitually bike. "Today I swim" is kind of implied to be future tense. As for "I am swimming", thats an action in progress so it would be "Oyoideiru/oyoideimasu".
But "I am swimming today" can also be an implied future, maybe even a bit more flexibly so than "I swim today." For instance, you could say, "I'm swimming tomorrow and the next day," and that only provides information about those two days. "I swim tomorrow and the next day" would be more likely if the swimming those two days was part of a pre-established pattern of swimming.