So you're travelling on Sea Dreams' ferry 'Symi' .
10:00 from Akandia quay in Rhodes Town port. Look for the white boat with the red stripe. In the same general area will be the 'Panagia Skiadeni' of Dodekanisos Seaways (Blue and orange), so if you see that first, the Symi won't be very far away.
Enjoy your stay on Symi island!