Hmm, water damaged not good for mains supplies...
I would suggest an open-frame 24V adjustable supply might be the best option - has to be mounted
in a case of course, but you should be able to get to around 28V on the adjustment, and they are available
in a large range of power outputs.
Quick search found this one at Farnell: http://uk.farnell.com/xp-power/ccb200ps24/power-supply-medical-ac-dc-24v/dp/2783921
Only 5% adjustment range though, but 25V and 8A ought to be usuable?
Simple PWM duty cycle control might be enough, at either a low rate (1Hz?) or with an LC filter you
could run at a high rate (ultrasonic). The LC filter smooths the load seen by the supply if switching
rapidly.