A walking spirit

Mark 6:49 "49 But when they saw him walking upon the sea, they supposed it had been a spirit, and cried out: 50 For they all saw him, and were troubled. And immediately he talked with them, and saith unto them, Be of good cheer: it is I; be not afraid."

I reckon that the disciples had these superstitious beliefs such as of spirits walking around influenced by greek teachings regarding the state of the dead.
Look at the word used in this verse for "spirit":

spirit: 5326. φάντασμα phantasma; from 5324; (properly concrete) a (mere) show (“phantasm”), i.e. spectre: — spirit. (Source accordance)

i reckon

You got this right.