i might be wrong here... but if memory serves me...
blue sinking symbol = friendly ship sunk
red sinking symbol = enemy ship sunk
green sinking symbol = neutral ship sunk
grey sinking symbol = a ship has sunk but the cause of its sinking is unknown or was caused by weather.
the grey usually happens when you damage a ship, which later sinks but it is beyond range of your sensors.
im not sure if you lose credit or not... i think it just depends.
the best thing to do is at lease stay within hydrophone contact of a damaged ship.
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