So the map construction the easiest I can figure out anyway. One sky box so all is contained in one map. Top level is the ships. You can see and shoot each other from each ship. You portal to the next room. In a 3d view it would be below the water or maybe a small cloud town above. Portal over and fight the next thing we can create plus the other team. If you die you spawn back at the boat and start your trip over. If you find the next portal you travel on. My guess is we will have to hide these room under the water. In the end I figure it will be like freeze tag in the fact that the first team to control the last level will win.
Problems is room sizes we can only make a map so large so it will only fit so many rooms. Portal exits going to need more than one each of they will just be camped at and the game will die at the first portal exit.
We can create death wall traps and traps portal of death LOL portals that send you right back to the begining and so on.