Freshwater feeders will cause fatty liver disease in saltwater fish, so I agree that they should be your very last resort. Puffers can be SO picky, but just keep trying different things until you find something he/she likes.
I feed mine a mixture of mysis shrimp, Formula Two algae pellets, clams, raw shrimp, and freeze-dried krill. Puffers also like mussels, scallops, squid, etc., so try all of those first (I know, our fish eat better than we do!)
Also, try soaking the food in Garlic X-treme or something similar. Garlic can often entice picky eaters to eat.
Good luck and let us know how it goes