Summary: By "wet works" Podesta was referring to personnel changes in the Clinton Campaign after the disastrous 20 point loss to Bernie Sanders in the New Hampshire primary that day.
In an email that John Podesta sent to Steve Elmendorf, he said:
"Didn't think wet works meant pool parties at the Vineyard"
Since this email was sent four days before Justice Scalia's death it is being promoted as evidence that Scalia was assassinated. By all accounts, including Scalia's family, he died of natural causes. But what of this small piece of evidence? Is that true that Podesta could only be referring to an assassination?
Look at the context of the email. Elmendorf talks of a "rough night", and the date is Feb 9, 2016. That's the day of the New Hampshire primary, which was won decisively by Bernie Sanders, finishing 20 points ahead of Clinton. That's obviously what the "rough night" refers to. The most likely thing that "wet works" refers to is the subsequent bloodletting within the Clinton Campaign - i.e. firing people. Described in the media as a potential "bloodbath".
The term was also used by Keith Olbermann
"Wet works" or just "wetwork" is actually an a euphemism for murder or assassination, however the usage is not limited to that - it can also mean tough or distasteful work. While it's not a common phrase, a brief search of the internet shows several usages that do not involve killing people. In fact it most commonly is used in the context of staff reorganizations with terminations.
Notice the other similar words there: "rough up", "kneecap", "dispose of". Clearly these are meant figuratively, as it's just an article about encouraging people to speak up in boardroom debates. The reference is about getting rid of team members who are not working out. Just like with the Clinton campaign.
Again, "wet work" refers to getting rid of staff by firing them, not killing them.
Again this is referring to staff changes, terminating people.
It's not limited to that usage though:
Even the Conservative leaning Fox News has used the term similarly, in a political campaign context.
"Wet work" has also been used in the sense of political work that does not look good, for example Richard Nixon's early years in politics:
I don't think they are suggesting Nixon was assassinating people for Eisenhower.
And in similar but more obscure usage referring to political (non-killing) dirty work.
And here's the Koch Brothers, their "wet work" seems to be the Super PACs and targeted ads.
Some more obscure usage:
Even John McCain had lawyers doing "wet work" for him.
And of course, Trump:
Bob Woodward used the term "wet work" in his book "The Price of Politics"
And a minor clarification of the quote, "the Vineyard" is a common term for Martha's Vineyard - a popular retreat for the rich and powerful, and a place where Clinton had been many times. Note the capitalization.