I mainly use my float boats on medium sized rivers with flows from 150 cubic feet per second to about 2500 cublc feet per second. A cubic foot is equal to 7.48 gallons.
I also use it to fish small lakes and ponds. I use a front and rear anchor to keep my boat from blowing around. Sometimes I will use it to anchor me in a deep river hole and sometimes to slow me down. I use ten lb. pyramid anchors and in big rivers I use a 12 pounder.
I haven't had much time with my kayak yet. I've only used it on lakes/reservoirs. I look forward to trying it on the river, but haven't gotten around to that yet.
According to friends that fish from their kayaks, a 5lb folding anchor is all I need to hold the kayak in lakes. That worked for me last fall.
That's all they use in rivers, too, but I can't vouch for how well they'd hold in a decent current. Might depend on bottom composition.
I mostly only use my float tube where "power boating" is not allowed. Nebraska has many lake that have a 5mph speed limit or a "no wake" rule in effect on it. This just makes it easier to concentrate on fishing and kicking than having to worry about the waves created by boats. I've taken my tube out on lakes where the rules allow power boating, and it can be interesting to say the least.
Like Alex G., I look for lakes and rivers where motorized boats are prohibitated. There are a few state parks close to where I live and the scenic part of the New River where tubes/pontoons are safe and fun to use.