Enter these codes on the Cheat Code screen (under OPTIONS.)

ALLDLURES      All lures available
WHEREDFISH     Less Fish 
SILLYSOUND     Different Sounds
GIMMEDVIEW     Better View
NOCOMP         Remove Other Fisherman
WHATADRAG      Slow Boat
HYPERBOAT      Super Fast Boat
BAGDSNAGS      No snags while fishing
NOPENALTY      No penalties in tournament mode
MONDOFISH      Huge Fish 
ALLDLAKES      All Lakes 
ALLDCASH       Lots of cash 
SUPERLURE      Easier to catch fish
IWINIWIN       Automatically win tournaments
FISHMAN        Lots of fish on fish finder 
SUPERSTRING    Unbreakable string 
HAPPYFISH      Makes fish more active 
GIMMEDFISH     Fills live well with large fish
RUBADUBDUB     Turns boat into a bathtub 
HEADADBIGA     Big headed fisherman