The first one, should run whenever a unit enters a region (the water), if you have several regions for water, you should be able to add them all to the events. When they enter, add them to a Unit Group (a global variable), and reduce their Energy Regeneration to 0%. Think this can be done with the "Change Unit Property" action.
The second trigger should remove them from this unit group when they leave the region, and also return their Energy Regeneration to whatever it was before.
The third trigger, running every 1 seconds, use the "Pick every unit in Unit Group", and reduce their energy (also with "Change Unit Property". Or if you prefer that, use an If, Then, Else action to check their energy, if it's above 0, reduce it by however much you want to reduce it, or if it's at 0 already, kill them.
If I understood you right, this is how I'd do it.
You'd need three triggers.
The first one, should run whenever a unit enters a region (the water), if you have several regions for water, you should be able to add them all to the events. When they enter, add them to a Unit Group (a global variable), and reduce their Energy Regeneration to 0%. Think this can be done with the "Change Unit Property" action.
The second trigger should remove them from this unit group when they leave the region, and also return their Energy Regeneration to whatever it was before.
The third trigger, running every 1 seconds, use the "Pick every unit in Unit Group", and reduce their energy (also with "Change Unit Property". Or if you prefer that, use an If, Then, Else action to check their energy, if it's above 0, reduce it by however much you want to reduce it, or if it's at 0 already, kill them.