well, the consistent polling wouldnt be expensive, its either a blank page for no move by the oppenent, or actual co-ords if they have made a move, thats using php of course.
basically, i thought of it this way, you set up your ships, your oponent sets up their ships. the data is sent to each (that way as soon as incoming co-ords have been sent you both immediatly know if its a hit or miss and less data sent in the long-run), then when you send co-ords its sent to the server. While you are waiting for the opponent to make their move you check the page every 2 seconds or something, if the page is blank it means your oponenent hasnt sent any co-ordinates, and as soon as co-ords appear the game can stop looking at the server, you then make your move, sending data to the server and so on until all ships are sunk.
thats my theory anyways :wink: