

Castle Rock sits on Agincourt Reef, a ribbon reef on the outer edge of the Great Barrier Reef accessible only by boat. The reef structure runs from the shallows down to around 24 metres, with visibility commonly reaching 15 to 30 metres in the crystal clear water. Currents are variable, so check conditions before you get in. Beginners will feel comfortable in the shallower sections, where sea turtles, giant clams, and schools of fusiliers and surgeonfish are regular sights. Snappers, sweetlips, and trevally cruise the mid-water, while whitetip reef sharks rest on the sandy bottom. Stay clear of lionfish, and keep an eye out for jellyfish near the surface.
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Visit between April and November when weather and water conditions are most reliable, and aim to get in the water early in the day before wind chop builds on the surface.
The site is located on the outer Great Barrier Reef, approximately 65km northeast of Port Douglas. It is accessible only by boat, with numerous tour operators running day trips from the Crystalbrook Superyacht Marina in Port Douglas. The boat ride takes about 90 minutes.
Specific accessibility options depend on the tour boat operator. Inquiries should be made directly with the chosen company.
Stinger season (jellyfish) is typically from November to May, and protective suits are recommended. The outer reefs can be subject to strong currents, including down-currents near walls.
Depth
0-25m
Visibility
15-30m
Skill Level
Beginner
Entry
Boat
Boat required
Water Temp
23-30°C
Current
Mild
Typical Dive
40 min
Best Time
Apr, May, Jun, Jul, Aug, Sep, Oct, Nov
Castle Rock (Agincourt Reef 1) requires Open Water certification at minimum, though Advanced certification is recommended due to the site's depth range extending to 24 metres. The dive site is located off Port Douglas, Queensland, and is suitable for beginners in the shallower sections. All dives are boat-based with professional operators offering guided tours.
Castle Rock features regular sightings of Malabar grouper, whitetip reef sharks, and green sea turtles alongside schools of trevally and fusiliers. Divers commonly encounter snappers, sweetlips, and surgeonfish cruising the reef structure. The site's mild currents attract diverse fish populations throughout the year.
Castle Rock (Agincourt Reef 1) ranges from the surface to a maximum depth of 24 metres. The varied depth profile makes it accessible for Open Water divers whilst offering enough depth for Advanced divers to explore deeper sections. Visibility typically ranges from 15 to 30 metres across all depths.
The optimal diving season at Castle Rock runs from April through November, spanning eight months of favourable conditions. Water temperatures during these months range from 23 to 30 degrees Celsius. These months offer the most consistent visibility and calmest sea conditions for reef diving.
Castle Rock is exclusively accessed by boat from Port Douglas, Queensland, with no shore diving available. Multiple dive charter operators depart daily from Port Douglas marina, offering guided tours with equipment rental available. Facilities include showers, toilets, and dedicated dive shops at the departure points.
Primary hazards at Castle Rock include jellyfish during stinger season, venomous lionfish, and occasional strong currents despite generally mild conditions. Divers must remain alert for boat traffic in this popular area and maintain respectful distance from whitetip reef sharks and other marine life. All dives are conducted with professional guides who monitor conditions and marine interactions.
Castle Rock is classified as a beginner-friendly dive site with Open Water certification required. The shallow sections starting at surface level allow new divers to build confidence whilst the mild currents provide manageable conditions. Dive operators provide guided tours specifically designed for less experienced divers visiting Agincourt Reef.
A 3mm wetsuit is recommended for diving at Castle Rock in Port Douglas. Water temperatures range from 23°C to 30°C.
Castle Rock in Port Douglas has a maximum depth of 25 metres.
A typical dive at Castle Rock in Port Douglas lasts approximately 40 minutes. Actual dive time depends on depth, air consumption, and conditions.