117/88 is really not that bad of a BP. I bet if you checked it a few more times and took an average, you would be at the stand 120/80. Mine fluxuates like crazy.
As far as dehydration, it can lead to increased BP because your blood would be thicker, probably in extreme cases. You really are close to he norm though, Id be curious if you went back to your gallon a day and got checked if it would make a difference. You should test it out