Correct spelling, explanation: the correct form is beaches because this is a plural noun of beach. The singular form ends with ch sound which means we need to add an e before adding s when creating a plural form. This is why beaches is the correct form.

Definition of beaches:
noun, the plural form of beach, an area with sand along a shore
The beaches in our city are breathtaking!
Due to a storm, all the beaches in this area are closed.

Incorrect spelling, explanation: this is an incorrect spelling as there should be an e before s in the plural form is beach (hence beaches). This is due to the rule that if a noun ends with s, h, sh, ch, ss then we should add es to create a plural form (for pronunciation reasons). This is why beachs is an incorrect form and the correct one is beaches.