Packed away, with both hands free. That's the crew's instruction on most ships and it's the right one. An expensive camera should never hang from your neck while you're stepping between a moving ship and a moving Zodiac. Once you're seated and the driver says it's safe, get it out. Before you approach a landing or come back alongside, pack it again.
Use a strap or tether while shooting, but one that doesn't create a hazard. Cross-body distributes weight better than a neck strap. Keep it short enough that the camera can't swing into a Zodiac tube or another passenger. Never attach a tether to emergency equipment or anything the crew might need to move quickly.
The framing worth holding onto: the photograph is never more important than safe boarding. A camera can be replaced. A fall between a ship and a Zodiac can be extremely serious, and it happens in the two seconds when someone is thinking about their lens instead of their hands.