Pool hours vary and are at least 10AM to 10 PM, extending to as much as 8 AM to 12 AM during busy times. Stormalong Bay Pool will have lifeguards during the pools' operating hours.
I don't have a definitive posting of pool hours listed on the Beach & Yacht Club FAQ thread because they change throughout the year, and sometimes even in the same week at the discretion of the recreation management based on weather conditions and crowd level.
The answer of at least 10 am to 10 pm is a good one, although in the summer months, expect a 9 am opening. I have personally never seen reports of an 8 am opening. I think 11 pm would be the typical closing time in the summer, not sure if the pool still stays open until midnight occasionally if conditions warrant it.