Time is enough but you have short time inside the park. The park is open at 10am. So, you can take airport express to airport bus to Central. If you get there early, you can get around a little while. Take bus 260 around 9:30am at Central exchange Square and get off at HK Country Club. Then walk several minutes to the entrance of Ocean Park. I think you should prepare to leave at 1pm, maybe earlier which depending on the way you get back. Anyway, I will not recommend this.