Apart from the usual hurdles of surviving the cold weather and being surrounded by predators (including hawks, two Jack Russells and the neighbour's cat) the biggest problem will come once they are hatched and take to the water.
Firstly, they are thrown out of the nest after about ten hours, long before they can fly. Sadly, when they enter the water, there is no way they can get back onto the pontoon, which is a good 12 inches above the water level. We could build some form of floating ladder which floats on the water and gently slopes up to the pontoon, which would help them get back to dry land, but this cannot deal with the other obstacle; the tide. Where we are, the tide flows passed the boat at up to 5 knots. The first time they go into the water they will be carried away and lost as even a fully grown bird struggles against the flow. A weak duckling just learning to swim won't stand a chance.
I am fairly sanguine about nature, but this is all very sad. I was so looking forward to seeing them hatch and develop, but now it seems that even if they overcome the local predators, as soon as nature takes them onto the water, there is little chance they can survive. A dreadful waste of time for the mother. I cannot help but wonder if there is any natural parental bond between a duck and her ducklings, or am I applying human emotion to what a bird will simply accept as part of nature.