The Texans could not get back to Houston after Saturday night's preseason game in New Orleans and flew to Dallas to wait out the storm.
Watt posted a message on social media, urging everyone in Houston to stay safe.
"I'm sitting here watching the news and checking the internet and seeing everything that's going on with Hurricane Harvey and the damage it's causing back home," Watt said. "It's very difficult. It's very difficult not only because we have family and friends back there, some guys have young kids, some guys have wives and families.
"But that's our city. It's very tough to watch your city get hit by such a bad storm and not be there to help. Not be there to help with the recovery. Not be there to help with the process. It's very tough."
Watt upped his goal of $500,000 in funds for flood relief after raising $200,000 in two hours. Watt personally gave $100,000 to the fundraiser, tweeting the message, "Your support is phenomenal!"
The Texans will remain in Dallas on Monday and will hold practice at The Star, the Cowboys' training facility in Frisco.
The team said no decision has been made on plans for the rest of the week or Thursday's preseason game against the Cowboys, which is scheduled to be played at NRG Stadium. The Cowboys have offered to host the game, and the league will make the final decision on where the game will be played.
"Everybody in Houston stay safe," Watt said. "We're thinking about you. We're going to come back. We're going to help you out."
Detroit Lions safety Glover Quin, who played for the Texans for four seasons, lives just outside Houston in the offseason, and his wife, Gladys, and their children are still there. The 31-year-old said Sunday afternoon that his family is safe and their home is all right, but the streets and property around them have been flooded.
That has led to some tense nights for Quin, who said he has been up until 3 a.m. or 4 a.m. watching coverage on The Weather Channel and staying in touch with his wife to keep her calm.
"You're watching the news and seeing the city flooded and places that you've been and areas that you know, like, 'Wow, oh wow, that water's really high,' because you can recognize the area and spots and just seeing overpasses where the water is just up," Quin said. "It's sad, you know what I mean. Behind our home, we have this big field, and somebody owns some land, and they've got some animals, some wildlife as well. My wife walked out to the fence to take some pictures of the water behind our home and said she was hearing stuff.
"She didn't really know what it was. Just, like, hearing and then she paid more attention to it, and she said, 'Man, I've never heard animals cry like that.' Just nowhere to go."
As of now, Quin said he is scheduled to go home Friday if he can fly into either of Houston's airports, but he said he would leave the Lions to go home earlier if he is able to get back to the city. He is not expected to play in Thursday's preseason finale in Buffalo and said he has had conversations with Lions coach Jim Caldwell about what has been going on.
"Yeah," Quin said. "I'll go back right now if I could."
Caldwell said earlier Sunday that special-teams coach Joe Marciano, who has a home in the city, said his house is OK, but a lot of homes around his have had damage.
ESPN's Michael Rothstein contributed to this report.