Now, I know that cactus can't normally be placed like so:
[a][b][a]
[b][c][b]
[a][b][a]
A=Air B=Block(Any) C=Cactus
But for some reason, you can place cactus on sand underwater. It also doesn't break!
Is this a bug?
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[media]
I think so.

Confirmed. (Assuming you mean that it doesn't break when placing underwater; placing a block next to it or punching it breaks it like normal).
No long possible under water or in lava.