It is OFF LIMITS to proceed north of the High Bridge due to the Zebra Mussels infestation. The National Park Service boats you see are enforcing the restrictions. The National Park Service has a station just north of the High Bridge on the Wisconsin side of the river. Here is the link to the boating rules for the St Croix River. I recommend printing a copy and keeping them in your boat.
Boating Rules for the St Croix River
As far a getting bait while on the St Croix. Beanies sells bait, they are located across from Hudson just south of the I-94 Bridge. You can also get bait at Jimmies while on the river – just tie up to the shore near his bait store and it is a short walk up and across Highway 95 to his bait shop.
About fishing upriver of the High Bridge. You can launch your boat upriver from any of the access points above the Stillwater DNR Boomsite launch (William O’Brien, Somerset Landing, Copas, Osceola, Franconia, Interstate Park) and come down to Stillwater. You just cannot launch from downstream and go up past the High Bridge. It is kind of a pain to launch above and come down but it is worth the effort. That area above the High Bridge does not get much fishing pressure. A word of caution – at normal pool elevation, like right now, it is pretty shallow running upriver and you should proceed with caution. One other thing to consider: you are restricted to SLOW SPEED, above the Arcola Sandbar. Read the Boating Rules for a definition of SLOW SPEED. It takes you a while to get anywhere if you follow the rules.