That is unrealistic to get a sub to 503m, since the hull would have crushed when you are under 350m (just a appoximation) (for the best U-boat in WWII, Type XXI only. The figure for other boats such as Type VII is lower)
But you're lucky
"Life's like a box of chocolate"
Anyway, to surface the boat, you need to use compressed air to push out the water in ballast tanks. In deep sea, its hard to do so since water pressure is too high and prevents water from being displaced. That's why you cannot surface again
Don't be sad since you are luckier than 3/4 of sailors
in Battle of Atlantic