Two things will factor into whether you'll use a pool in January:
1. if the pool itself is in the sun for most of the day, it'll usually be warm enough. For example, we have stayed at 3 homes in Coral Bay and being on the west side of the harbor means the mountain keeps the sun from hitting the pool all day; it is gone by 2-ish. So if your plan to to be at the beach, then have a dip later, it'll be cooler.
2. where you are coming from determines what you consider warm...us US Northerners can handle colder waters, so the pool doesn't have to be that warm. Those who are used to warm ocean water will not be as comfortable.