You can just leave the water as-is with no worries.
As for swapping out with an airlock, that is purely a personal preference. The blow-off tube is technically an airlock, so you are really only deciding if you want to leave it alone with a large airlock or switch it for a small airlock. Either way will work.
"If guns don't kill people, why do we give people guns when they go to war? Why not just send the people?" - Ozzy Osbourne
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Canon T2i | EF-S 18-55mm | EF 28mm 1.8 | EF 50mm 1.8 | EF-S 55-250mm
3 whipped dog straights, Gillette Ball End Tech, 1959 Gillette Fat Boy
Secondary: Strawberry Melomel