puns - I don't get this joke. Is it some kind of play on "water, too?"

I don't get this joke. Is it some kind of play on "water, too?"

Transcript:

Two scientists walk into a bar. The first says, "I'll have some H2O." The second says, "I'll have a glass of water too. Why did you say H2O? It's the end of the day and there's no need to talk about work." The first scientist stares at his drink, angry that his assassination plan has failed.