How are the fluid resource amounts modified?

Fluid resource amounts scale with the Resource amount (200x the amount).

Why was the "Ice island" map generator resource added?

Aquilo map generation works a little differently: islands are placed where resources are generated.
So if the No resources preset is selected, no small ice islands would be created on Aquilo.
This resource setting will generate islands without resources, producing vanilla-like map generation.

What effect will it have with other map presets?

If active and left at its default settings, it will generate a few empty islands on Aquilo.
You can experiment with the setting in the Map Generator using the Preview option.