Welcome to the boards, Kagato! May your steel stay sharp and your spells cast true throughout the Gael Serran.
Yep, that's really frustrating... happens to me all the time. Ya know, if I knew fish were going to be that hard to kill in the water, I'd have blown them up...
Unfortunately, there's no good alternative (besides perhaps dropping your resolution down). I have occasionally had better luck when I rotated around slightly... and when I get a good attack spot, I keep swinging until the critter is dead, dead, dead...