One main obvious solution is to discontinue the use of synthetic fertilizers because they are the leading cause of hypoxia, due to their source of nitrogen and phosphorus. Discontinuing the usage of these fertilizers is hard and unlikely for farmers because they are very dependent on them. One helpful and small solution for this is to lower the use of the fertilizers and only use as much as the soil can observe.
Another very effective way is to use natural sources of nutrients, for example manure. Also using farming methods such as no-till and contour cropping. These solutions reduce runoff because they allow soil to absorb more water and nutrients.
Another solution is to restore coastal wetlands. This provides a natural filter and helps remove excess nutrients from the water before it reaches the ocean. We can all help out with preventing ocean dead zones, weather by refusing the use of fertilizers or even just a small portion of cleaning up trash and liter to help keep the ocean clean.