Does drinking plenty of water successfully prevent hemorrhoids?

Maybe. Only in so far as drinking plenty of water reducing constipation would i expect to see a benefit there. If you aren't constipated it is unlikely to help. Constipation and straining on the toilet, ie anything that increases lower abdominal pressure can precipitate hemorrhoids. Hemorrhoids are simply dilated veins around the rectum.
No. Hemorrhoids are caused/worsened by anything that increases abdominal pressure. Constipation, obesity, pregnancy, weight lifting are a few examples. Drinking more water is likely to decrease constipation and may help prevent hemorrhoids, but there are other factors involved.
Maybe, with fiber. Hemorrhoids are related to straining to pass bowel movements (stool), with the increased pressure about the anus allowing the surrounding veins to herniate. Drinking plenty of water, along with at least 25 grams of fiber daily, will increase stool bulk and liquidity, making stool easier to pass and potentially reducing the incidence of hemorrhoids. Worth a try.
It helps, but.. Drinking plenty of water can help prevent constipation, which can contribute to hemorrhoid formation. But, you also need to have enough fiber. Exercise and adequate nutrition are also factors. As with most conditions, a good healthy life style goes a long way to prevent problems.
Probably not. Water by itself will just keep you peeing - however - have a bowel of fiber one with a big glass of water - now were talking!
Not likely. Hemorrhoids tends to be due to two factors--constipation and lifestyle. Obesity, chronic alcohol abuse, pregnancy all lead to a higher incidence of hemorrhoids. Constipation, due to the need to strain during defecation, increases the pressure on the hemorrhoidal veins.
No but it helps. Hemorrhoids happen because of straining to pass hard stool. Drinking water mainly helps general hydration of the body. Fiber, however, actually increases fluid inside the bowel to make stools softer and easier to pass. There is less straining with soft bulky stools and less hemorrhoid development.
Probably. Your colon is a water reclamation plant for your body; it reclaims the water in your intestines. If you drink plenty of water, your colon does not have to reclaim as much water resulting in softer stools. Softer stools often results in less hemorrhoids. However, this is just one of many possible factors for hemorrhoid formation/prevention. Other factors include fiber, GI health, stress, etc.
Sometimes. There are many factors that contribute to hemorrhoid formation. Constipation is only one of those factors. However, for some people that can be a primary factor. In these people, preventing dehydration and constipation by drinking plenty of water can help prevent hemorrhoids. Usually fiber supplementation is needed as well.