Every summer, my dad teaches a science class about sea
turtles in the atlantic ocean.

Once we capitalize the correct words, the sentence becomes:

"Every summer, my dad teaches a science class about sea turtles in the Atlantic Ocean."

We should capitalize "every" because it is the first word of the whole sentence. In English, as well as in many other languages, the first word of a sentence should always be capitalized to indicate the beginning of the sentence.The words "Atlantic Ocean" should also be capitalized because they are proper nouns, that is, they are the name of a certain ocean. In English, we should always capitalize proper nouns. Take a look at the example: I wanted to see the ocean. ("ocean" is a common noun, not specific) X The Pacific Ocean has calm waters. (it's a specific ocean, its name is "Pacific Ocean")Many people will be tempted to capitalize "sea turtles", but there is NO reason to do so. "Sea turtles" can be seen as a common noun that refers to a species of animal. It is the same as saying "birds" or "dogs". It does not specify anything.

In conclusion, we capitalize words at the beginning of sentences. We also capitalize proper nouns, that is, names that specify something.

Learn more about nouns here:

link