Laying mines is a bit different in SH4
 

As I found out when I tried to place a minefield in a test mission.
After a few tries, I finally got the hang of it and put a nice big
minefield in a shallow passage in the Phillipines. I made a Jap
AI convoy and proceeded to run them thru it. I thought it was
the 4th of July when those AI ships got tangled up in the minefield.
Like watching the movie "In Harms Way".

JIM (RAdm. Rockwell Tory)

...you're sick! I love it! :D My ships usually stop at the edge of the minefield unless they're too deep.

I put the mines at a height of -1 which meant they were just below
the surface. You could not see them from a top view, had to go
under for a look see....

JIM