This weekend I ran two playtests of my game; one with 4 players on a 4' square board, followed by a second game with 2 players on a 2' square board. We used the Fistful of Lead rules system and both games worked very well.
In the after game wash-up we decided that the optimum size for a 4 player game was a 3' square board, due to the slow movement of dive suits and the reduced weapon ranges underwater. My next major item for this project will therefore be a 3' square game surface (although I will still be working on more figures and terrain for this project in the meantime).
Some pictures of the games (thank you to Dewbakuk for supplying two of these):



