Question: You often *plunge* into the sea or *plunge* you hands into your pockets.

Can the word be used to describe a "horizontal immersion"?

Example sentence:

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> She opened the door and **plunged** into the blinding sun.
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If not, what's a more appropriate word?

Answer: 
It's perfectly acceptable, but you might want to consider the "literal baggage," for lack of a better word, that the figurative term carries with it.

Take the OED literal definition that this derives from:

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> To thrust, throw, or drop into or in a liquid, penetrable substance, deep pit, container, etc.; to immerse, to submerge.
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In any case that you use the word "plunge," readers will associate it with the literal meaning to derive its figurative meaning. If you want to invoke feelings of falling deep into something, then it's appropriate. Just be aware that the use **ordinarily** refers to falling or pushing *in a downward direction*.
"Plunged" is a good word to use, but "sunlight" would be much better then "sun". 

Otherwise, you could say "emerged" into the sunlight. 

One historical example shows both "plunged" and "emerged" would be good choices: 

[Reports of Observations of the Total Eclipse of the Sun, August 7, 1869](https://books.google.com/books?id=c00QPyT2IjMC&pg=PA108&lpg=PA108&dq=%22plunged%20into%20%22%20sunlight&source=bl&ots=WqTGBpgo99&sig=zznbtkA2vX7fQjSBLuwgE1CF9vY&hl=en&sa=X&ved=0ahUKEwjT0aK25vvTAhUF6yYKHaqOBnkQ6AEIUTAI#v=onepage&q=%22plunged%20into%20%22%20sunlight&f=false):

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> ...observed with what frightful rapidity we were plunged into the dark shadow, and the contrary effect as we emerged into sunlight
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