Something waterproof, and smaller than you think. A compact dry bag or water-resistant daypack of about 10 to 20 litres handles spare gloves, sunscreen, a water bottle, camera gear and the midlayer you took off. A roll-top dry bag or internal dry sacks protect electronics far better than an ordinary backpack. A full hiking pack isn't necessary, and some people just use parka pockets.
Check whether your operator supplies a backpack or dry sack before buying one. Several do.
In the Zodiac, keep it compact so it doesn't protrude, avoid loose dangling straps, and keep your hands free. Ashore, never put it down on the ground near wildlife.
Keep it clean for biosecurity too. Bags with mesh pockets and heavy Velcro take longer to clear at the pre-landing check.