I think you need at least two, a large one/longer blade and a shortie. Dexter Russel's are hard to beat, there are two grades-get the better one. I picked up a Cuda on sale at Bass Pro and it's nice. The small Rapala is about right for spadefish and the like but you'll certainly need more blade for a redfish or a king. I bought a Knives of Alaska Steelheader from Haywire that suits me well especially for thicker scaled fish like striped bass or reds.
A fillet knife needs to fit your needs and it's not easy to tell just by holding one in the store. When I'm on the pier I'll usually have three fillet knives and a steel, a couple of bait knives that I may grab for rough work and at least two pocket knives. Can't have too many..

