Dry ice remains low only if it's colder when exiting the machine than the ambient room temperature. If you want it to stay low, turn the room heat up a few degrees and it should hang lower for longer.
The basket doesn't need to be submerged in the hot water to boil off the co2... as long as the bottom layer is submerged, you'll get fog, and for longer.
Submerging all the ice will get you more fog, but for a much shorter period... you'll have to experiment to find the 'sweet spot' you require for your application, but I found a few cm of the basket submerged worked well on a stage show with a similar machine...
Remember that CO2 is a suffocating gas... Make sure there's somewhere for it to go once it's done the job of looking cool or your audience may start getting headaches/passing out etc etc