Turtle Ship No. 1 sinks right before Yi Soon Shin and the court officials' eyes immediately after launching. Yi Soon Shin is blamed for building a "defective" ship. In contrast to Yi Soon Shin's failure, Won Gyun is shown training an impressive troop of marines in the next scene.
In real history
Turtle Ship No. 1 didn't sink under Yi Soon Sin's watch(In fact, Yi Soon Shin lost zero ships during his 7 year war), Won Gyun sank it in his first and last battle 6 years later with the rest of 138 ships in the fleets.
Why the distortion of history? This series is based on a pair of anti-Yi Soon Shin novels, was often dubbed "Immortal Won Gyun" by critics, and the producers and script writers are beginning to show their true colors now.
Just remember how far the producers will go to make Yi Soon Shin weak and morally corrupt(sleeping with the daughter of his lover).