Silent service was a great game! The Commodore 64 version seemed the best to me. I have even considered getting another C64 from ebay to play the game again. Somehow, I don't think it will be the same.
Still, Silent Service had great 'game play'. Stalking a convoy, playing hide and seek with escorts, moving only at night, it was really a strategy game wrapped in submarine graphics (which weren't so great). Few games have had that much of a lasting impression on me.
Regards,
Malta