Orberg Cap XXIX

This the scene in Cap XXIX when the sailors are confronted by Arion whom they tried to drown.

Arion repente cum fidibus ornamentisque cum quibus se in mare iecerat apparuit.

Does this translate as: ‘Immediately Arion, with the beautiful lyre which had been cast into the sea by them(selves) (quibus se), appeared’ ?

Not quite. Some tips:

ornamentis is a noun, not an adjective.
se refers to Arion.

oh yes… with the lyre and jewels with which he was thrown into the sea..

se iecerat is active, not passive.