In an ideal world, they would have been able to get close to the island, and put the cubs in the water for the mother to take. That would have been the safest route. Because the mother was nowhere to be seen, they couldn't take that chance.
Dropping the cubs back into the water was as good as drowning them if she wasn't there to help. The other option was to pull right up to the island, get off, and put the cubs on the land.