Misconception #1
Water > Gaara sand.
No where in the manga is this the case. Even if that was the case Gaara has over come weaknesses such as Oil and Gold. Water gets absorbed by sand which make it's heavier. This doesn't automatically make it a weakness as this can be used offensively.
Take for example crocodile from OP.
Crocodile weakness is water,
BUT at the same time anything that contains water is also advantageous to him. If he gets hit by water it is a bad thing but if he hits something that contains water, such as a human being that would be a good thing. Also realize that the water by itself doesn't take down the sand defense it has to be followed up with another attack. GSB has zero destructibility feats nothing suggest it would get past Gaara's defense.
Water making sand heavier isn't something Gaara can't over come. During his fight with his Father Rasa he over came gold dust. Rasa buried the sand beneath them with gold but Gaara was still able to bring the sand up despite Gold being heavier. He accomplished despite using regular sand(not the sand from his gourd).
Misconception #2
Gaara requires chakra infused in the sand in order for him to control it.
I made an entire thread on this matter.
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TL

R Gaara doesn't require chakra in the sand to control it, chakra in the sand only makes the sand more potent.