Below are two variations of a sentence. Sentence 1 is clearly a compound sentence with two independent clauses, and the accepted rule is to put a comma before the conjunction (as here). Sentence two appears to be a simple sentence with a compound predicate. Some editors advise putting a comma before the "and" that connects the second predicate. Is that correct? What is the reasoning behind the inclusion of a comma there?
On warm summer days, I swim in the cool ocean, and I take long walks on the shadowy forest trails.
On warm summer days, I swim in the cool ocean and take long walks on the shadowy forest trails.