Is your Fish Hat dead or Alive? It’s the eyes that decide! And that give your fish its unique expression. Fish Hat knitters came up with a lot of great 'eye-deas'. Like these: 

The Originals:
cut two small circle of felt (as small or big as you like). Sew them on the fish. 
Dead: Embroider an X-es on each for a death fish. Or a dot for a living fish. 
Alive: Emboider a dot. 

 Great variations:
Groggy eyes:

Your fish just got a bang on the head or smoked something it shouldn't

Blinky the Three eyed fish:
You can see this creature sometimes in the Simpsons. Blinky is a mutant fish and a product of nuclear wast of Springfield Nuclear Power Plant. Make your own Blinky by knitting an orange fish hat and adding three felt eyes.

More variations:
Crochet eyes:
Crochet a flat (or not so flat) circle. Forgot how to? Look here:

 Knitted eyes
Naomi (Matataki at Ravelry) came up with these great knitted eyes. 

 Other great looking eyes:
- Pick two eyelike buttons from your button box;
- Place two buttons on top of each other;
- Use doll's/teddy's eyes (craft shop).