I started in a tube and a canoe. running play parks is always a good place to start. Golden, lyons, boulder. Apple valley, chutes, filter plant were all runs i did. Then started up the chain to things like Shelly's cottages, bridges, rustics, foxton, waterton, fractions, upper eagle, lower eagle. Then moved on to IVish stuff like Numbers, Upper mish, poudre park, lower clear, dumont, upper CC and so on and so forth.
I swam a bit in the beginning and really put some flat water time into getting my roll. Then battle tested it in play parks, alot.
Best thing to do is boat, every chance you get. Be serious about getting better, it won't happen unless you push it. There is a fine line between pushing it and knowing your limits, it is okay to walk. You don't want to end up having lost all confidence.