Having had some time to do a bit of growing, the ducklings were ready to get in the water.
The water is only shallow and they're supervised at all times so that that there is no risk of drowning. Whilst adult ducks take to swimming like "a duck to water" (pardon the pun), baby ducklings have not yet developed their waterproof feathers, so can become waterlogged and tired, and thus drown. It is however important that they learn to swim and enjoy the water, as they will spend the rest of lives in a out of a pond and bathing in water.
These guys seemed to have no trouble, and will now being going in for more dips more regularly.