An absolutely tiny plugin (only 2 lines of actual code), but nevertheless it's game-changing in some situations.
Basically it will simulate the friction of water. In vanilla minecraft, an arrow throws you back just as much while in water as while on land. A bit unrealistic and very annoying indeed when fighting those pesky skeletons!
Now, in water the punch will be about halved, meaning when you are walking towards a skeleton while in water, you will be stopped, but not thrown back.
That's it really.. In my opinion it's a bug of the game.. but eh now you can just install a tiny plugin.
Just drag the downloaded .jar into your plugins folder.
That's it, no configuration or anything