Looks to me like the “whiteness” could possibly be a reflection of your white background on the water. Is your white background at an angle? Try angling your white background up, so that when the flash hits it, the light bounces up and away, not back down onto your water. I see you’re still shooting at ISO 1600, is there a reason for that? 🙂 One of the first photos you posted had the exact same settings, but you used ISO 200 instead of 1600. You’ll have to turn up your flash some to compensate.
I popped your photo into GIMP, and with about 30 seconds of levels adjustments, I got this: (I didn’t do anything to the WB, just moved the black and white sliders). You should be able to do the same thing in Lightroom, try moving the Blacks slider to the left, and the Whites slider to the right.
You may want to just go back to using the on-camera flash, if that works better for you. There’s no law that says you have to use an external flash. My theory is that you use whatever gives you the best results, regardless of whether it looks/acts/feels professional. 🙂
