Watermelons are sometimes hollow in the center. Not a major problem just a little annoying. Melons and related fruits will sometimes form with a hollow interior or 'hollow heart'. The flavor is sometimes not up to par either, now that's a tad more than annoying.
Watermelons develop with a hollow center for a number of reasons.
1. Nutrient Deficiency
2. Irregular Watering
3. Poor Pollination
1. Excessive Nitrogen is good for the lush foilage and long healthy looking vines. The melons however, will grow quicker than the interior can form resulting in poor seed development and hollow centers.
2. Low boron levels in the soil can also lead to hollow watermelons. Boron is needed for seed and cell formation and fruit set as well as other factors. Boron being highly soluble is easily leached in some soil types, sandy soils in particular. Watermelons use boron to develop healthy seeds, when a healthy seed zone fails to form a hollow cavity sometimes takes its place.
Watering is critical to producing quality watermelons. The key time to water is from the time the fruit begins to form until it is half-grown. Excessive watering too close to harvesting of watermelon will reduce the sugar content and cause the fruit to have hollow centers.
Inadequate pollination is another cause of hollow melons, it is frequently accompanied by misshapen and or deformed watermelons. If weather was poor at pollination time, chances are your vines did not get as much traffic from bees and butterflies as they should have. Poorly pollinated plants will produce fewer melons and also substandard ones. Seeds also fail to adequately form, same as in boron deficiency. Inter planting a diversity of plants that attract bees in the vicinity helps to attract pollinators and reduce the risk of inadequate pollination.