Using mosfets as discharge loads is a pretty common technique, though I would still go with a aluminium heatsink over water.
Even though the mosfet he is using is rated at 2500w at 25c, it goes down with 20w per celsius up to 150c, when discharging into water that is boiling the internal temprature quickly goes above 100c.
Essentially he is pushing the fet way beyond what it is built to handle as it can't get the heat away quickly enough, a copper or aluminium heatsink would solve a lot of those problems.