How about having the ships cross the screen at different speeds (but not quite so many of them)? It would be interesting to see if you could also have the ships avoid each other on their way across the screen (give them some vertical movement just to avoid collisions with each other).

It would be nice if 'End' could be programmatically found and set; and have the 'Stack/Queue' switch remove only the water, so the user would not have to re-enter the islands again (and hope it is at least close to what he/she had before), only changing them at their discretion.

Some segments end up hiding (not supposed to); so, 600 arrives before the grid is actually filled. Still trying to solve the problem (any help would be appreciated).

Not sure I understand -- using the menu to reset the game or switch game works fine at home, but the uploaded scenario seems to want to 'stop' when these are used in 'Greenfoot'.