Best visibility comes at slack water, when the current eases. Pick your destination, check the water temperature, and get the next three days' calm windows to snorkel.
Pick a destination and enter a high tide, then press Show snorkel windows.
Current runs fastest mid-way between high and low, and eases to a near-stop ("slack") close to the tide turns. We mark the ~1.5 h either side of each high and low as the calm window — that is when visibility is best and you are least likely to be swept along. Avoid snorkelling in mid-flood or mid-ebb, especially on spring tides.
Above 24°C: comfortable in a swimsuit. 20-24°C: a shorty wetsuit helps. Below 20°C: you need a proper wetsuit and short sessions. These are long-run averages — check the current forecast before you go.