Here is another one by Steve Cartwright. I'm really enjoying his work and games lately.
He uses the same sort of structure for his code. The structure and game play to an extent is similar to Megamania and Plaque Attack.
There are 10 RAM bytes not used. I wonder if they were for a feature that got removed before release. There are also some bytes or tables in the ROM that aren't used or referenced.
If you're up for a learning project; consider hacking this to award the player higher points if they shoot an obstacle that is facing or traveling toward them. Another option would be to allow the player to shoot the Patrol sub.
*** 113 BYTES OF RAM USED 15 BYTES FREE
*** 4 BYTES OF ROM FREE