Hey Adrian:
Welcom to TONMO! If you have set up an octo tank remember to enter it into the OCTO Database on the main page.
I feed my bimac almost exclusively on crayfish. (I get them from my local fish store). He loves them and does not want to eat anything else.

I have found that they will only live for several hours in saltwater. Thus you should keep a close eye on them and remove them from the tank if your octo doesn't eat them before they die.
You will likely find that a bimac will jump on the crayfish almost immediately once you put it in the tank. I try to feed Tralfaz once every other day. I also feed at the same time of day every time. he now expects to be fed and will come out and start to beg as soon as I enter the room. Thus, he is expecting the crayfish and usually snags it before it even hits the bottom of the tank.
You are lucky that you have a free source of food close by so why not take advantage of it. There are also several threads about feeding octos within this topic. You may want to check them out. Good luck.
George