Divers new & old can sometimes get lost in the nomenclature of diving & when divers go on about diving “the slack”, they’re often referring to the tide.
The Ebb Tide is when the tidal waters are moving away from shore, when at their lowest shore divers refer to this as the “low slack”.
The Flood Tide, also commonly known as the Flow, is when those same tidal waters make their way back towards the shore. Once at their highest, they are referred to as the “high slack”; this minimal exchange of water is the favorite time of day for shore divers to head out.
This obviously depends on the locality of the divers, their objectives & the weather, & without giving away all the answers to the SoD’s exam, I hope this gives you better insight to why divers, sailors & Debbie Harry preferred it when the tide is high!