It's how a ship of around 200 passengers gets everyone off under the 100-ashore rule. One hundred people land while the other hundred go Zodiac cruising, then halfway through the slot they swap, which the industry calls a hot swap. Both halves get roughly equal shore and Zodiac time, and importantly nobody is sitting on the ship waiting their turn.
There's a detail here that changes the arithmetic. Passengers out on an off-ship activity don't count toward the 100 ashore. So if 20 people are kayaking, 10 paddling and 10 snorkelling, that's 40 out separately, leaving 160 to split 80 and 80 before the swap.
A few ships in the 113 to 116 passenger range use exactly that to land effectively everyone at once.
When weather cancels the activities, teams improvise. Two Zodiacs go the long way round while 100 people land.
The split model works up to about 200 passengers. Above that a ship drops back to roughly one site a day.