Contact your yeast supplier and ask them to reccomend a strain and fermentation schedule to achieve the desired results. At 6% you should still be able to pull some higher alcohols and esters appropriate to a high gravity ale. Warmer fermentation temps can accomplish this. Yeast strain is very important, I'd go for an estery English strain over a clean American Ale strain for example. If you do blend with water follow everyone else's advice and consider adjusting the PH of the water with a food grade phosphoric to maintain correct final Ph in the beer. At small amounts of water its probably not an issue but at 25% depending on your water supply it may swing your PH to high.
"You are what you is." FZ