Try the shops on Wan Chai Road, the stretch from opp Tai Yau Bldg corner on Johnston Road heading towards Urban Council Wanchai (Wet) Market at junction with Queen's Rd East. Lots of fresh fish, some on ice - you'll have to point out the mackerel if you don't speak Cantonese. I'm sure they can clean it up for you. Parked 20 min in a car opp one such shop (near junction with Tai Wo St), was witness to some quick and clean cut-up jobs and 2 or 3 "other jobs" which were shall we say not for the squeamish (this desire for freshness is all very well, but I wouldn't carry it to this extreme).