When you go ashore in your allocated zodiac, do you have to stay with that group or can you wander around on your own and then return to the ship on any available zodiac?
Sorry, I see that I didn't really answer the gist of your question. What happens when we land varies. Where we have to walk though penguins and seals, on landing we shed the life jackets and they ask us to gather in a group with one of the expedition leaders; the group doesn't have to be your zodiac. We then have to stick with that guided group until we return to the life jackets and the zodiacs. Return on any available zodiac.
Sometimes that happens outwards but we are free to wander back at our own pace.
Sometimes its entirely free ranging - the Falklands was like that (except on the particular guided tours at/around Stanley).
I think in Antarctica it will be more 'stay in your guided group' due to number restrictions and hopefully abundance of wildlife.
On this cruise 'zodiac groups' seems to be most about having people arrive at the mud room in an orderly and staged fashion. After that, you might be on any zodiac outwards or inwards or in any group onshore. Also, its almost impossible to stage it so you get your particular favourite Exped leader and/or zodiac driver. Some are definitely better than others both in knowledge and in how they handle their zodiac. All competent drivers of course, but the one I had yesterday was
constantly (every two seconds) putting the engine in and out of gear, either forward and reverse from neutral, when we were paused to take pictures. Every time it jarred the zodiac and your picture was compromised. I was going to mention the issue to her at the end, but the re-boarding was difficult and I didn't get the chance.
Another zodiac driver/Exped leader was identified early on by several people (including me) as being very abrupt to the point of being rude. But more recent reports are that he was OK; someone was going to mention the issue to the Chief Expeditionist; dunno if they did or if that changed the guy's attitude.