In accordance with the special requirement of determining the external gravity field quickly in ultra-large sea area, a detail analysis is made on the applicability and limitations of the three traditional models, i.e. spherical harmonic expansion model, direct integral model and point mass model, for computing disturbing gravity. And then three modified computational models, i.e. modified direct integral model, modified point mass model and mixed model of direct integral and point mass, are proposed one by one. A numerical test has been made to prove the reasonableness and validity of the suggested models. It is shown that the singularity in three traditional models have been eliminated in the solutions of the modified models. They can satisfy the practical needs of determining the local gravity field quickly in ultra-large sea areas and full height. The accuracy of the modified point mass model is estimated to be about ±1mGal in smooth sea areas, and to be less than ±3mGal in rugged sea areas. The obtained conclusions are valuable to the practical engineering projects.