The makeup of the bottom should be considered as well. Especially the areas where the boats will be put in will get trashed in a few hours. Would consider a small concrete pad or raised platform for launching ships. Same goes for battling sides of the pond, they will get torn up with all the captains. Just some random thoughts for what it is worth...
You can make the islands part of a bridge and allow folks to step out onto the larger island.. that is what we were thinking of doing. Plus lots of smaller islands would add some unique battling as the channels would be around 2ft to 3ft wide.. and such.
I wouldn't go much deeper than 4'. Beyond that you are getting into swimming / searching for sunk boat territory.
