The 105lb model of the Tsunami water heavy bag is for martial artists who need resistance for their strikes. Lighter models have a tendency to swing and hop when struck hard, especially by a fighter that commands a lot of body weight behind their strikes.
In case you're worried about leaks with a water filled bag, you should know that Tsunami's double rubber bladder system has been tested and approved for serious conditioning work. The impact of strikes gets absorbed by the outer shell and dispersed throughout the bag, leaving less impact to get redirected back into the striker.