Re use indirect lighting is only useful when you have already rendered the scene once, and wish to reuse the indirect lighting solution. It won't work if you changed your lighting conditions or if you render at a higher resolution. What happens when this is checked, is that the radiosity calculations and prepass are not computed. It goes straight to rendering the scene, using  what(s stored in memory. Maybe radiosity and infinite procedural terrains (displaced water) are hard to compute together? Will try something like this on my lkaptop, which has about the same specs as your machine.