The ramp just below the 494 bridge in South St. Paul is a good bet for this time of year. It is usally ice free unless we get a few really cold nights. A couple guys from the board do an excellent job of keeping it clean. This access puts you close to a few prime winter fishing holes. Also Hidden Falls upstream by the dam stays open but I have never used this one in the winter.
I work right next to hidden falls and stop by regulary and as of thur. the ramp looked as if it were summer out. Guys have been putting in their rigs and fishing up by the lock and down river at the confluence of the minn. river and the miss. Depending on the type of boat you have you can run down river about 10 miles or so (through down town st. paul) and fish the areas accessible by the 494 ramp. Hope this helps and good luck.