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Each wreck has a primary slack — the main, reliable slack-water window, given as High Water Portland plus or minus the wreck's offset. That's what the planner uses by default.

Some wrecks also have secondary slacks: lesser or conditional windows that are weaker, shorter, or only work on certain tides. The planner ignores these unless you tick “Include secondary slacks”. When it does plan a dive at one, that dive is flagged “secondary slack — use judgement”.

Example — Salsette: primary slack is HW+4h; the secondary slack at HW−2h is neaps-only and not fully slack.

Secondary slacks demand extra care: confirm the tidal conditions (neaps vs springs), brief the divers, and always verify timings yourself before diving — slack windows shift.