Water hazard is the chapter with the most sightings of the G-man in the entire series, with 4 sightings in total. Why did he appear so much in this part? Perhaps because he was worried that Gordon would die because he was facing too many Combine forces too soon.
4
Because Gordon is actually a terrible driver, especially with airboats; hence why G-Man has to keep an eye on him so much in that chapter.
Or perhaps the G Man man likes the scenery of drained water, toxic waste, and industrial buildings.
I find Water Hazard to be rather anomalous in terms of design, it's a bit unlike the rest of the game and it is especially unlike the Episodes. Some of the game's most obscure lambda caches are located in Water Hazard, if you never managed to get the Lambda Locator achievement, chances are that you missed at least one in this chapter, you can find them in places that are very much out of the way, going after these caches often feels like going out of bounds, you're ignoring your usual Half-life 2 instincts and doing weird things that Valve normally doesn't ask you to do in their games. There's also all of the strange optional stuff you can do, places where you can park your airboat, hop on top of it and collect ammo that is so hidden away, that probably less than 0.5% of the playerbase even knows they are there. There are also spooky hidden skulls placed by mappers for you to find, from a time before achievements were a thing. There's that strange pitstop you can do at one point, in that barn-like place where you can recharge your health and suit and then lower an elevator to drive off with your airboat afterwards, it very much feels like something Valve would skip if they remade the game with the experience they gained since then, because it's weird. That's the keyword for Water Hazard: it's weirder than most folks realize, we normally just speed through it and don't notice how many peculiar things can be found in it, and I guess G-man's excess of sightings is another one to add to that list of anomalies. I think that's part of why it's one of my favorite chapters in HL2.
He's monitoring Gordon's sanity during this poopy ass chapter