A multicategory sea ice model that explicitly takes into account redistribution of undeformed ice into categories of ridged and rafted ice is introduced with the intention of investigating the simulation of the open water fraction within sea ice. The performance of this model is compared with that of a reduced model that accounts for only two categories, the latter type of sea-ice model being most widely used in global climate models. Two idealized scenarios are considered: polynya formation along a coastline under offshore winds and a low-pressure system passing over an ice field. Furthermore, the two models are compared under more real-world forcing conditions as they exist in the Weddell Sea. In simplified situations, both models produce identical results. Upon introduction of more realistic ambient conditions, however, the multicategory model produces larger open water fractions than the two-level model. Concomitantly, and consistently with earlier findings, the multicategory model produces thicker ice. This is a result of the larger horizontal gradients of ice motion which, in turn, are due to the weaker ice strength that arises from a higher abundance of thinner ice. This leads to more open water as well as increased dynamical growth of ice. The open water fraction is of relevance in global ocean circulation models, where the amount of open water influences the deep- and bottom-water formation rates such as in conjunction with Antarctic coastal polynyas.