Ironically, no pain, no gain. So any workout youryou're bad at generally becomes a great cardio workout. As Carl Foster, a professor from the university of Wisconsin, once said during a presentation:
When you want to work out, don't go cycling on this:
Instead use this:
The point is, working out isn't about how fast your bike goes. Its about how fast your heart is racing and your lungs are ventilating. So when you say you're a poor swimmer, it might actually be a decent workout, since its guaranteed to cost you a lot of energy.
As for a good workout for plantar fasciitis, you could try cycling as this is said to be a great rehabilitation exercise when you have a plantar fasciitis. Other alternatives would be going to the gym or something like rowing.
Just a small note: I do find it strange your plantar fasciitis keeps recurring, even when normal walking normally. Besides if theythe orthotics don't take away the problem, don't forget to go see the specialist again, because it might not be working the right way.