It depends, what do you want the water to do? Simplest is just to apply a hydrostatic pressure load, in which case you don't need to model the water at all. If you define in more detail what you are looking for it is more likely someone can assist you.
Try to use CEL (Coupled Eulerian-Lagrangian) method of modeling. There are a lot of tutorials in youtube. The main idea in your case is that the dam will be modeled using lagrangian bodies (where material is connected with the mesh like in most usual simulations), but water will be modeled using Eulerian domain (material of water will flow through the non-deformable mesh). Material of water is modeled using EOS (equations of state) material models (Us-Up model is used in most of cases). Good luck.
I can certainly understand that, you want to model WATER, but in the software platform usually, we model "WATER PRESSURE" instead of "WATER" directly. Having said that, it depends on what you are exactly trying to do by modelling the water.
On the other hand, in some particular case, water can be considered as an incompressible and inviscid material. For this case. an effective method for modelling water in ABAQUS/Explicit is to use a simple Newtonian viscous shear model.