As deep as you can dig is good if you want to break the ship there and get its pieces back ensured.
if you want it to never hit the ground, then you need to dig it as deep as you can dig when waves roll out in big storms. I know you said no swells but when the storms come, even then? If not, its good to go.

I don't know how you could possibly bury a lit fire. IT would smoke itself out or something.