SO, the locust planned to sink Jacinto. Speculation here has been running rampant about how it might have been a trick and all that. There is one very simple solution to why it was in fact a desperate tactic, not a trick: The Hammers of Dawn.
In the original plan, while probably taking heavy casualties, the Locust would survive the Hammers. Command is linked to Jacinto right? So in the event of Jacinto sinking, command would go down with it. And without command, the Hammers would go offline.
However, if the humans knew about the big sink, command would evacuate, the hammers will remain online, and when the horde (or what's left of it) emerges, they will be hammered to hell and back.
And as such, discounting a massive oversight by the writers, the locust could simply not afford the humans pre-emptive knowledge of the sinking plans. Humanity may have Lost Jacinto, but the locust will have lost all protection against the Hammers. And without protection against the Hammers, the horde would be wiped out.