You could go for a program such as Paint Shop Pro and a plug-in such as Virtual Painter, but you won't get such good results as with PhotoArtMaster (and I'm a big Virtual Painter fan). There are "watercolour" filters for most image editing programs but they aren't a universal solution and work best when combined with other effects.
If you want to put time into learning something well, then get Photoshop Elements (user-friendly, but eventually you'll run up against its lask of functionality), Paint Shop Pro (less user friendly, but powerful), or Photoshop (great but expensive, and can seem intimidating) (there are other programs too but these are the ones of which I have experience) and follow along with the examples on this and other sites. As time goes on you'll find plugins for Photoshop or Paint Shop Pro that will help you achieve particular effects. But this is a big learning curve. (If you go down this route I'd suggest taking one or more of the classes at
www.lvsonline.com which are really newbie-friendly).
But if you really want something simple to learn where you can get great results quickly then I think PhotoArtMaster is your best option. It's pretty userfriendly once you get used to it, although the interface isn't immediately obvious and the trial version where the majority of the options are greyed out doean't exactly encourage you to explore. The full product does come with a manual that has several step-by-step examples in it, and if you work through those I think you'd feel much more comfortable with the program.
Similar effects are available from the same company in a plugin called Buzz, which lets you create filter effects in Photoshop that can be combined with other filters and blending modes to produce more flexible results. But that's more expensive and puts you back into the steep learning curve mentioned above.