Underwater visibility and current strength are the two most variable and impactful conditions for recreational divers. Understanding what drives them — and how to respond when conditions change unexpectedly — is an essential planning and in-water skill.
What Affects Underwater Visibility?
Visibility is determined by the amount of suspended particles in the water:
- Plankton blooms — Common in cooler, nutrient-rich waters. Can reduce visibility to less than 5 metres but often bring dramatic marine life activity.
- Silt and sediment — Disturbed by currents, boat traffic, or divers with poor buoyancy. Avoid fin-kicking near sandy or silty bottoms.
- Tidal movement — Incoming tides often bring cleaner oceanic water; outgoing tides can stir up silted lagoon water.
- River run-off after rain — Freshwater run-off from land carries sediment and can drop visibility dramatically, particularly near river mouths.
Diving in Currents
Ocean currents are caused by tidal movement, temperature gradients, and wind-driven surface circulation. They can be a diving asset (drift diving) or a safety hazard (unpredicted current change during a dive).
- Drift diving — Divers intentionally use current to travel through a site with minimal fin effort. The dive boat follows the group using an SMB or diver flag. Excellent for covering large reef areas.
- Up-current and down-current — Vertical water movement. Up-currents can cause uncontrolled ascents; down-currents push divers deeper despite buoyancy attempts. Both are serious hazards in specific sites (certain passes and channels).
Reading Conditions at the Site
On arrival at any dive site, observe the surface for 60 seconds before entering: direction of floating debris shows current direction, surface chop indicates wind strength, and dark patches indicate deeper water that may have different currents below the thermocline. Your divemaster briefing should include expected current strength and direction — if it does not, ask.