The fleet in my setting are forced to leave the planet due to a volatile virus, and leave in pre-prepared spacefaring vessels capable of transporting ten million people per ship. The ships have algaculture-based farms for sustenance, but food isn't nearly as important as water. Water can be recycled from urine, but urine can only be recycled so many times before it loses all of its benefits. Are there chunks of ice in space that the ships could collect and use for water? The ships could come with a massive tank of water to keep everyone hydrated for a lengthy amount of time, but it obviously wouldn't fully be recyclable.
EDIT: Can urine be infinitely recycled for water? I was under the assumption that less water content and more waste content would be made each time urine is recycled.
The ships are equipped for travel through both interstellar regions, and within solar systems. The main goal is to find another planet with permanent living conditions that suit their needs. Another Earth-like planet if you will. There's obviously not another Earth-like planet anywhere near our solar system, so these ships are going to be long term living conditions until another Earth is found in a far away galaxy.