Buried in a sea (of data) - mixed metaphor?

This seems to me like a case of what I like to call "editor syndrome", which could be summed up as "I am employed as an editor, therefore I must edit things (whether they really need to be edited or not), in a sort of long winded parody of "Cogito ergo sum". I take a different stance on the issue - in fact I'm rather fond of mixed metaphors. I certainly wouldn't qualify it as "wrong". PS: I also agree with everything Jason posted above.

First "to be buried at sea" is a standard way of disposing of a body. So that is not metaphorical.

Without any difficulty at all, and using the search term "buried in a sea of", I found the following:Buried in a Sea of Dead Woods; Oh To Be Buried in a Sea of Wildflowers! the Tehama, as seen from the mountain tops, appears buried in a sea of white cloud; Elizabeth Song | Buried in a sea of paper; Gas drilling leaves villages buried in a sea of mud. The metaphor here is "sea" - a very large quantity [of something], the metaphor is not "buried".

That said, there is nothing wrong with "buried in a sea of data", but you should give the actual example that was criticised because not all uses are idiomatic. Your image: "I imagine something buried in a sea of confetti-like data." is not idiomatic. I should be either "I imagine something buried in a sea of confetti like data" or "I imagine something buried in a sea of confetti like data."