If your humidity was below 70% at the start of hatching, that's too low. Also do NOT crack the remaning eggs until day 30 at least. I've had them hatch just fine several days after the first one - on day 32! They may still be viable, esp if the last candling showed movement and slower development. Slow development indicates a cool incubator. Shrink wrapping means too dry. I fight low humidity all the time, its usually about 20% humidity here in the air. I often have to run a humidifier in the room with the incubator, plus fill all the trays and use wet sponges to get over 55%. It definitely affects my hatches.
I would NOT float test them, you could easily drown a hatching egg. I had some in the back of my incubator one time that, after I took the turner out for hatching about day 27, sat just a tiny bit low and had a little shell in the water. They didn't make it. As far as I know the float test is used on old eggs you might want to eat - the older the egg, the larger the air cell and the more likely it will float. Incubated eggs should have a large air cell and might float, but I wouldn't think it means the duckling is alive.
One way you can follow if your humidity is right is to mark where the air cell is at the beginning and check each week. The air cell should get bigger and be about 1/4 of the egg at hatching. If it gets big too fast, up your humidity, if it stays small, lower your humidity. I also use a good kitchen scale to weigh my eggs every week when I candle. They should lose about .1 ounce a week as the air cell grows. Of course that varies by the size of the egg to start with! My big duck eggs weigh up to 4 oz at the beginning (Silver Appleyard), the little ones about 2.5 oz (Khaki Campbells). Most range from 2.75 to 3.0. They will lose about 1/2 oz in 4 weeks.
For more information, check "Storey's Guide to Raising Ducks" by Dave Holderread. It is a very complete book with lots of incubation information from one of the top experts in hatching waterfowl.
