Morning is not a preference here, it is the answer. The creeks and sounds are near flat at first light, the sea breeze usually fills in through the late morning, and by mid-afternoon in summer there is a short chop on the open water that makes tubing punishing and waterskiing pointless. Anything towed behind a boat should be booked as early as the operator will run it.
Wind is the variable that cancels. Parasail operators have firm limits and will stand down when it blows, often at short notice on an otherwise sunny day. Jet ski tours continue in more wind but become a wet, bouncing ride once it gets past about fifteen knots.
Tide matters less than for kayaking but is not irrelevant. An outgoing tide running against an easterly breeze stacks up a short, steep chop in Calibogue Sound and Port Royal Sound, which is uncomfortable on a ski and unpleasant on a tube.
Season: water temperature runs from the low eighties in July and August to the upper fifties in late winter, so the motorised season effectively runs April to October, with a shoulder where a wetsuit top is welcome. July and August are hot, busy and thunderstorm-prone from mid-afternoon. September and early October are the quiet sweet spot with warm water and thinner crowds.
Remember the state rule: no personal watercraft between sunset and sunrise.