Why does water weaken ion ion attractions? My lecturer has told me that water, having a high dielectric value, will &quot;shield&quot; ions and reduce ion-ion attractions.

I really don't see why. Having water molecules surround these two ions doesn't mean that Coloumb's force doesn't exist anymore - nothing in $F=\frac{kQq}{r^2}$ has changed just because water molecules are surrounding them. I recognise that the water molecules will arrange to face the ions in a polarised way (oxygen to the $+$ ion and hydrogens to the $-$ ion). However, I don't see how having a shell of water surrounding you will reduce the attraction. I guess it would prevent them from coming closer due to the sheer physical size of the water shell, but wouldn't there still be attraction?
Can someone explain to me why the ion-ion attraction decreases when water is there?
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.
$$F = \frac {q_1 \cdot q_2}{4 \cdot \pi \cdot \epsilon  \cdot r^2}$$