I think one of the best options is to paint the interior a light color. I have a white boat with a white interior and white (plastic) seats to boot and don't have a problem with the Arizona sun. The white interior can be a little bright at times, and I've heard that light blue is probably the best for heat/glare.
Another option would be to get a few of those throwable square flotation devices. They're not really soft cushions, but they would make sitting on the metal benches a little more comfortable. Along those lines a padded stadium seat would be a good option too, or perhaps one of those garden kneeling pads.
I think one of the best options would be a bimmi top for shade, but that's probably a $250-$300 upgrade.
You could also look into seats; plastic is cheaper than vinyl, and probably cooler too. Bass Pro, Wal-Mart, and many online retailers have the clamp on plastic seats and they're usually only $25-$35.
Hopefully those ideas give you something to think about. :mrgreen: