Great game
This was really fun to play. It had a lot of polish to it, the idea was pretty original (for a "defend your..." at least) and I really liked the music too. (Maybe it's just me but I thought it was catchy as hell)
Only point of criticism is, as others have said, that certain bubbles seem overly powerful and others seem far less useful, even for what they are intended. Metal bubbles in particular are a bit cheap as others have pointed out, especially since they're the 2nd ones you get. Also poison bubbles and shrinking bubbles seem to do basically the same thing (kill divers). I think if you did a few simple things the balance would be much better: 1) switch metal and explosive bubbles 2) change poison bubbles to "acid bubbles" which will destroy anything inside of them, not just divers, and if so, switch their place with the shrinking bubbles 3) make subs tougher, they're the last tier of enemies but aren't tough at all. Maybe by allowing them to get free of even metal bubbles, and requiring explosive/acid/shock-wave bubbles to lower their health and defeat them.