"κολυμπάει" is an older form. With a phonetic phenomenon called "κράση" that is contraction, nowadays became "κολυμπά". It happens when the accent of the verb is on the last syllabe and the verb ends in -αω, -εω , -οω in the Present of Indicative. Both forms are correct. Other examples: γελά, γελάς, τραγουδά, τραγουδάς, πετά, ξυπνάς...