Poisson - fish (sounds like "pwa-sonn". But this is simply a word covering ALL fish. You will notice there are different fish species on sale, and they all have different names in French.
Thon - tuna
Saumons - salmon
Merluches - hake
flétan - halibut
morue - cod
plie - plaice
flet - flounder
truite - trout
bar de mer - sea bass
dorade - sea bream
fruits de mer - seafood
crevettes roses - prawns
bucardes - cockles
moules - mussels