A question about a line in Good Morning, Vietnam

"Hot and wet", in a sexual sense, means the woman you're with is very turned on and ready to have sex. This is a very good thing.

"Hot and wet", in an atmospheric sense, means it's going to be difficult to breathe and there will likely be a high mosquito count, particularly if you're in an area where mosquitos live, such as a jungle.

He was trying to add a laugh to a situation that no soldier wants to be in, to try to take their minds off it for a few minutes. Their clothes are going to be soaked, there will be high moisture content in the air, the temperature will be uncomfortably high and they will likely be bitten by many mosquitos. That's pretty much as dismal as it gets. Add to that the fact that they're creeping through a jungle filled with heavily camouflaged enemies, and it pretty much sucks.

Jungles, more commonly referred to as rainforrests, are both hot and wet.

He's broadcasting to military personnel, which means he's talking to basically no one but men on his broadcast. As such, he's saying being "hot and wet" is fun if you're with a lady, but not when you're in the jungle, fighting the Vietcong in this case, and with very little chance of female companionship unless they're on leave, which they wouldn't be if they're in the jungle at whatever base they're stationed at.

He's making light of the situation the soldiers find themselves in by making a somewhat raunchy joke.