When I got to Yibukua in mid-July, the water level was high, at neck level. The water was very cold and the current moving quite fast(fast enough to sweep you away if you didn't hang on to something). River crossings are dangerous regardless of your swimming ability. Although I managed to make it across(detailed in my travelouge), I don't recommend you make the crossing when the water level is high. Since it takes a while to get across the river, there is an added hazard from North Korean border guards(I did not encounter any, but escaping them would be much more difficult with the river being so deep). There's also a risk of hypothermia from the cold river water(I was OK, though I did end up catching cold later on).
Besides the obvious danger of North Korean and Chinese guards spotting you crossing the border, you should watch out for locals, too. After I got back from crossing the border, a bunch of locals said they'd seen me coming over from the North Korean side and accused me of being a North Korean escapee. I managed to escape from the situation, but it was unpleasant and better avoided.
While it is easy to leap to and fro across the border, don't go too far into North Korean territory. You don't want to get into a foot race with North Korean border guards!
The North Korean sentries also do not like tourists taking photos of them, so use caution and a zoom lens when trying to get a souvenir snapshot for your VT pages.
Security at this area has increased in recent years, so now you not only have to be on the lookout for North Korean sentries but Chinese border guards as well. It's best to go in the winter time when the tributary freezes and more crossing points are accessible.
The enthusiastic local tour agencies in Dandong love touting this day trip to Chinese and South Korean tour groups, but are far more subdued when speaking about "Yi Bu Kua" to foreigners who can understand Mandarin.