The sea off the Texas coast is half a foot higher than it used to be, because of global heating. Hotter water expands to fill more space, and melting surface ice that used to be on the land mass of Antarctica and Greenland and is now in the oceans as extra water and caused the sea level to rise. (The level of the land can also be lowered by e.g. drilling for oil). When the sea level is higher, that allows higher storm surges, leading to extra flooding.
2. The increase in the surface temperature of the ocean over the mid-20th century by about 1 degree Fahrenheit increased the amount of moisture in the air above it by 3-5%. More moisture in the air means that when it rains, it really pours.
3. The water in the Gulf of Mexico is warmer than a few decades ago, and warm water feeds hurricanes the way anger feeds the Incredible Hulk. Harvey was significantly more intense as a storm because it developed over warmer water.