Tropical Hibiscus grown in very hot, sunny areas such as the Gulf Coast may struggle with heat & dehydration during mid-summer afternoons. In direct sunlight in temperatures of 95F or more, no matter how much you water, some plants may not be able to transfer water for the roots to the leaves fast enough to prevent wilting. Wilting is the most visible result of plant stress from lack of water.
In such hot areas of the country, it is often a good idea to provide tropical hibiscus with some sort of afternoon shade. If the plants are in pots, then it might be possible to move some hibiscus plants to areas where trees or shrubs can provide a little cover during the hottest part of the day.
Water often during very hot weather, preferably light watering once or twice a day. Watering less often, such as heavy watering every couple of days, leads to cycles of very-wet to very-dry, which stresses the plants. The goal is to keep the soil in the root zone just slightly damp at all times, which gives the plant the water it needs as well as keeping the roots cooler than air temperature.
Automated drip watering systems are great for hibiscus, as they provide regular watering that is delivered directly to the root area. Drip watering also avoids water drops on leaves and blooms, which can leave unsightly spots.