My advice - find what foods don't make you vomit, I went for anything that was easy to swallow but equally not too sickly as sickly or too chewy caused vomiting mid meal.
In terms of meals I stuck to plain food - mashed potatoes and veg with any meat I could tolerate, rice with plain things like chicken and veg, sliced bread but only if it was white.
In terms of snacks, I stuck to salt and vinegar crisps, cream crackers, mini cheddars, and for a short period I craved rice cakes but that craving quickly turned back to hating them like I always had pre-pregnancy.
I found it also helped to eat child sized portions for a while, the more I tried to force myself to eat the more I felt the urge to be sick and more often than not if I ignored the urge and continued eating I had to grab the sick bucket. Breaking up meals also worked for me in terms of getting my lunch down at work, so I sometimes just ate a sandwich at lunch and then consumed one item an hour of the rest of my lunch until home time. Just keep eating little and often.
You're doing the right thing drinking plenty, but have you tried fizzy water or perhaps adding some squash to fizzy water, its weird but it did help a bit with my nausea on bad days as did lemon drinks made with fresh lemons, and ice cold water. Ice lollies I also found were quite good for nausea.
And if it does not get better and you cannot stand the sickness any longer go to see your GP, I was getting so down about it I broke down on the doctor who felt like there was little option other than medication which was ultimately how I got through the weeks of sickness hell. It does get better, I know it might not seem like it now, I didn't believe anyone who told me that but it really does!