Exclamation marks in the middle of a sentence

Would this usage of exclamation mark be correct?

I want to — honest! — give you a slap!

Yes, I know, it's a quite bad example, but I wonder if this usage of an exclamation inside a sentence is correct. If not, is there any situation where an exclamation mark would be acceptable in a sentence?

I'll give you another example:

He said he ate a thousand (!) fishes.

Is this correct?

Exclamation points are generally for ending sentences. I think what you want to convey by putting an exclamation point in the middle of a sentence is emphasis, which is probably more appropriately done with italics, bold, or underline. However, I think both of your examples are acceptable for informal writing, and certainly understandable.

Yes - both examples are common. This is characteristic of informal use, and often seen in works of fiction.