By the way, crayfish taste very good. I went crayfishing with my nephew a few weeks ago. This time I slaughtered and gutted them, as soon as we got back. I cooked one for him, but he didn't like it (they do have a strong fishy flavour) so I took the rest home with me and froze them. I've just got round to cooking them today and they taste really good. The freezing didn't affect the texture or flavour.
Crayfish are basically little lobsters, and taste just like lobsters - i.e. wonderful, if you know how to cook them well. They shouldn't taste fishy. Are you sure they were alive up to the point when you cooked them? The moment they die they start going rapidly down hill.
I did kill and gut them before cooking them on this occasion. This is because I didn't have the means to purge them. They should be left in a tank of clean water for 24 hours, to empty their intestines before cooking, otherwise it's not good. I killed it by cutting the spinal cord and brain with a sharp knife behind the head, broke the anal fin and pulled it to remove the intestines. Then I cooked them immediately. I also took some home and froze them an hour later.
Taste is subjective. I agree crayfish taste similar to lobster, which I would describe as fishy, but you may have a different opinion.
In the past, I have purged them before cooking, but the flavour wasn't any different to this and it didn't seem to be any different to the ones I froze an hour after slaughter and gutting.
Actually the most humane way to kill a crayfish/lobster/crab is to freeze, rather than boil it, that way the nervous system shuts down, before death. It's also convenient as it can be later cooked from frozen, without affecting the flavour.