There are several dive operators operating from Mandraki Harbour, Rhodes Town: Trident Diving School (one of the best with many qualifications), Rodos Diving and Waterhoppers.  There may be others but they do not appear to have a location in Rhodes Town/the harbour.  No dive operator has a 'shop/office' in the town itself.

In the south part of the island, though, a brand new Padi certified dive centre can be found.  In the area of Pefkos, 5 minutes drive from Lindos, there is a brand new dive centre, "LEPIA", which visits several dive spots in Lindos and the Plimmiri ship wreck.

The best time to talk to them is between 5pm and 11pm when the boats come back from a dive to the harbour.  If they are not busy you may also be able to book a same day trip between 9am and 10 am when they are moored up and getting ready for the day's trip. Prices are very similar.  If you are not based in Rhodes Town most of them can arrange to meet you at the dive site.

The Waterhoppers Dive school is a 5* PADI school and has 2 bases on the island.  They have a boat excursions from Mandraki Harbour for all levels of divers and also take passengers and have snorkelling lessons.

Waterhoppers also have a dive office in Lindos.  It is 50 metres back from the main beach at 'free carparking 2'.  The office is open Monday to Saturday and offer qualified dives from Lindos and the wreck dive in Plimiri.  They also take beginners for diving from beaches in Pefkos, Kiotari and Kolymbia.  The day can be for the whole family as diving is available from age 8 upwards. age 8-10 are called 'bubblemakers' and are looked after very well by the team and have their hands held.  The diving session for beginners usually lasts 2-3 hours.  The office number for Lindos is +30 22440 33040.