I've never had to start sprouts/slips by placing SPs in water, it can become a stinky mess with nasty slime/mold rotting away at the potato. Periodically changing the water may help to hold down the growing mess. When I dig up mine in the fall I'll wash them off and store them inside on a multi-rack for future use. By about this time (Xmas) many will start sprouting slips without the need of water or anything else. By Spring they're loaded down with slips and I'll carefully carve off the slips and insert/plant them into old used cell packs with a mixture of seed starter and garden soil. Within 3 weeks they're ready to transplant outside. Plus I can still use the SPs that the slips were taken from. No mess, no fuss.

Likewise I have never planted sweet potatoes in a container or bags. But have seen pictures of those who have and they just let the vines spread out over them and down to the ground. And from the reports I've read they produce well. Production is very hard to predict. Weather conditions can cause production to change a lot. The same varieties I dug up to nine pounds in 010 from didn't produce a pound after the hail and the hot, droughty summer last year. And the one I planted in mid July produced several tubers but none as large as the one they were produced from. I would say the most I dug last season was around 5 pounds. The same variety that produced nine pounds the year before. So very hard to predict. And then again growing in a bag, container may affect production also. Jay

Sweet potatoes do not like cold ground. For this reason they should be planted after your soil temperatures have warmed up. planting them in raised beds with black plastic covering your beds will certainly help. Also selecting varieties that are early maturing varieties will allow you to plant a little later when the ground is warmer and still make a good crop. For rooted cuttings, 2 weeks or so will give them time to push out new roots. You may take your slips directly from the tubers or you can take tip cuttings from the vine tips. For rooted cuttings a 2" cutting with at least 2 nodes stuck in a half peat half perlite mix and put in a high humidity area under mist and preferably bottom heat. For unrooted cuttings your vine tip cuttings should contain at least 4 nodes (approximately 8-10" long) be sure to trim leaves back to a central stem. One thing to consider is that the tubers over several generations pick up viruses that affect your yields. For this reason all of our plants start out from tissue culture where they are screened for viruses.

I've never grown sweet potatoes, but maybe this link will help you. Here is a link that might be useful: sweet potato