Why does water weaken ion ion attractions?
Electrostatic field caused by ions or any other external source causes partial orientation of molecular electric dipoles along the field vector. Their orientation causes displacements of center of positive and negative charges. This displacement causes induced electrostatic field, that decreases the primary field intensity by the value of relative permittivity.
By other words, you replace the vacuum permittivity in the Coulomb law by the water permittivity.
$$F = \frac {q_1 \cdot q_2}{4 \cdot \pi \cdot \epsilon  \cdot r^2}$$