Let's look at your sentence: "A body covered in scales, a tail and fins make fish great swimmers."
This sentence is almost correct, but it is missing a small word that will make it clearer. Since the subject is a body covered in scales, a tail, and fins, these three things together make fish great swimmers.
To make the sentence grammatically correct, you should add the word with after fish. So the sentence becomes:
"A body covered in scales, a tail, and fins make fish great swimmers."
Actually, this sentence is almost perfect but it would be better to use the word helps instead of make because the subject is a body (singular), not multiple things. Also, it’s better to include commas between items in a list for clarity.
The improved sentence is:
"A body covered in scales, a tail, and fins help fish be great swimmers."
Or you can say:
"A body covered in scales, a tail, and fins make fish great swimmers." (if you think of the whole list as the subject)
Here is a simple tip: When you list things, use commas between the items, and make sure the verb matches the subject (singular or plural).