Edit CoreMaps.xml. Specifically the bit which goes
<WaterPercentageMin>20</WaterPercentageMin>
<WaterPercentageMax>40</WaterPercentageMax>
Need to do it for each map size. They all have different percentages...
Good idea to back up the original file first. Or make a changed file in ...\Documents\My Games\FallenEnchantress\Mods