Unfortunately, the damage the water gives to a player is determined by game files and not by code in the addons. In the addon, there is a simple boolean for whether the water hurts you or not.
I would recommend using zone events to simulate lava. So long as you set the zone big enough to cover the map, you can use onInZone events to do whatever you want to the player while they're in the water. Note that the surface of the water will be invisible with zones and you'll have to use bricks to replicate it.
