Hong Kong is a city of islands, over 200 in total, so there's always fresh seafood available...however, the abundance of seafood available does not necessarily translate to great quality as I find the seafood cuisine to have the largest hit and miss ratio in the city.  It's even more frustrating because these dishes tend to be a bit pricy compared to the other dining options.  The signature seafood dish is Typhoon Shelter Crab a garlic flavored, large crab dish best enjoyed with a group.  During the fall season hairy crabs are a popular menu item and you will see stacks of the crabs tied up and on display at various markets.

I have not explored Seafood restaurants in too much depth...not having a job means I can blow two days of dining on a single meal.
UNDER BRIDGE SPICY CRAB
405 - 425 LOCKHART ROAD
This place is good but expensive for the budget traveler, although it becomes a better value if you come as a group of at least with one other person.  The signature dish, Typhoon Shelter Crab, can run up to $500HKD depending on the season, weight of the crab, etc.