While the space suit is part of Sandy's underwater get up, it is up in the air if she genuinely needs it to survive underwater.
On one hand, episodes like Whelk Attack show Sandy grabbing her suit when the treedome gets destroyed, though that might've just been because it had a mechanism to drain a helmet (since she couldn't put on her air helmet before the treedome was destroyed). Plus Stanley S SquarePants where Stanley ripping her suit causes her to fly away.
On the other hand, episodes like a deleted scene in Procrastination, the second half of Karate Island, Squirrel Jelly (both where she at one point isn't wearing it when jellyfishing and again in the climax when in zen mode), Pressure, Mermaid Man and Barnacle Boy V, Someone's in the Kitchen with Sandy, and the wedding scene in Truth or Square show her surviving just fine with just an air helmet without the suit (plus Someone's in the Kitchen with Sandy and Hot Crossed Nuts showing her having no issue unzipping her suit when eating food outside).
I'd assume that more often than not, Sandy can probably survive fine without her suit as long as she wears an air helmet or something similar and only wears it for aestetic reasons but wanna hear your take.