I used This tool (the bottom one) to move my character (on a copy of the shown game) to 10km away. Then I drowned myself, popped back to my spawn, and the squid were still there... I walked around for a full minecraft day and night, just to test it. It appears squid don't despawn like normal mobs (all the sheep and cows around my house despawned).
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"If there be trouble, let it be in my day, that my children shall know peace." ~Thomas Paine