Well, I have to tell you it was worse in the "old days of VMK" when the pirate game would give you 15 credits and the hats still cost 1000 credits.
But anyway, the easiest way to make credits if you are not good at the games at all, is to find 3 friends and play "friendly pirates". Play capture the flag, and take turns winning. To maximize points, make sure each round both teams get the flag, and then take turns on who brings the flag back to base. That way you get about 121 credits when you lose, and about double that when you win. Play 6 rounds and everybody is guaranteed to get a Key, too!