An easier way to implement this might be something like this:
The map is set up to be a battle on two islands, each with a citadel and a few portals near the water, and a number of flags in the water that mainly control stats. All demigods can wade through the water, but while in the water, they can only use a few of their basic melee abilities and can't travel anywhere near as fast. Meanwhile, archers and minos from the portals would board boats that magically appear and take them across. The boats are quite fast, and demigods would have a hard time taking them out with their restricted abilities. And of course, the archers would be constantly shoot at the demigods as well, making things even more difficult.
Every portal on an island has a flag and is reasonably captureable, and would send spawned units at the former enemy citadel.