The sea closing

The Pharaoh saw the sea splitting. That was not enough of a miracle for him: he still led his army through. Suddenly, the sea began to ...

Is there a verb that would specifically describe the sea uniting/closing?

There is nothing wrong with "closed".

Before they could retreat, the sea closed in on them at Moses' command.

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... through the waves, and when Pharaoh's soldiers followed, the sea closed in again.

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That afternoon the sea closed in round us again.

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Then, first sinking back a little at the stern, I thought, she slid slowly forwards through the water and dived slantingly down; the sea closed over her and we had seen the last of the beautiful ship on which we had embarked four days before ...

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The ship sank down at the howling of the storm, And the sea closed over my lover's form.

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A few moments, and the sea closed over all, and nearly 500 souls were swept into eternity.

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The injury done to her was too extensive to save her, and at 9.50 p.m. the sea closed over the last of the German raiders in that vicinity.

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In your specific example, I would use the word collapse. It means to fall or crumble suddenly, or to deflate or sink rapidly.

So with your example something like this:

The Pharaoh saw the sea splitting. That was not enough of a miracle for him: he still led his army through. Suddenly, the sea began to collapse in on the army.

This answer is based on the assumption that the sea parting leaves a gap below the level of the sea, with water on either side. The water collapses because it rushes downward to fill the empty gap.

If you interpreted the parting of the sea as the sea receding away, leaving a flat plain behind, then when the water returned I would use a phrase like "the sea rushed in" instead.