Put 1 gallon of water and salt into an 8-quart pot. Bring to a boil over high heat. Put the remaining 1/2 gallon of water and the ice into a container and set aside.
Add the squash to the boiling water and cook it for 3 minutes. Immediately drain in a colander and transfer to the ice bath. Shock for 3 minutes, replenishing the ice if necessary.
Drain the squash and transfer it to 2 half-sheet pans each lined with 3 layers of paper towels. Pat dry with additional paper towels. Remove the paper towels and place the pans with the squash in the bottom of the freezer until frozen, about 1 1/2 hours.
Once frozen, transfer the squash to a resealable 1-gallon plastic bag. Lay the bag flat on the counter and, using a straw, suck out any remaining air in the bag. Label and return bag to freezer. Don’t defrost the squash before using.