Help with getting out of a job. please!?

I have been trying to get a summer job for a few weeks now and haven't seen any luck. About 2 days ago I was called by walmart (one of my top 3 places to work, I dont care what other people say) and scheduled an interview this Wednesday. I was stoked but this morning I was woken up by my Dad saying he got me a job with my neighbor's business. It is in the "hood" and its a business that flash freezes seafood (mostly scallops). It's pretty much a guaranteed job because I dont know how likely I am to get the walmart job. I was in the seafood plant and I was in the packing place for no more than 5min and I can't get that smell out of my head. I am supposed to start working at 1 today but I would do anything to get out of it. The owner is a good friend of my Dad's and I would hate backing out of this. I would want to work with walmart so much more than at this hell hole. Any ideas of how to get out of this predicament or am i stuck?