For pool & sun, eliminating places that are super-expensive like Four Seasons (since I'm not familiar with them), they might consider Intercontinental or any of the three major hotels in Admiralty.
IC's pool is huge. However, be warned that there is construction going on right next door; not sure if that will bother them. Doesn't bother me,but what does bother me about IC's pool is that I've found it to not be heated enough.
It's been a few years since I've been to the pool at I.Shangrila/JWM. I've been staying at Conrad more recently. Pools at those properties are probably all similar if memory serves. At least at Conrad, the pool is considerably larger than Sheraton's, more heated, but it does tend to get windy. Jacuzzi-wise, Sheraton >> Conrad.
Cordis, as mentioned above, has a nice rooftop pool. But it is partially/largely covered (for good reasons... it gets real windy up there). So I don't think it's ideal for sunbathing.

Sheraton's quite a bit cheaper than any of the Admiralty hotels, also a bit cheaper than IC which has constructions next door. Plus, while you are correct that their pool is small, it's still as big or bigger than some of the other TST hotel pools. Plus those twin (triple?) jacuzzis with harbour views are hard to beat. Perhaps Sheraton should still be under consideration for your friends.