If you are 'up to your neck' in something it means you are really busy with it or involved in it.
I'm really up to my neck in work. It's the end of the financial year and I have so many accounts to finish.
Tony owes the bank a lot of money. He's up to his neck in debt.
I'd love to join you this weekend but I really can't. I'm up to my neck in revision for my final exams.
Don't confuse it with
A pain in the neck. If someone or something is 'a pain in the neck', it is very annoying.
Your sister is a real pain in the neck. She's been playing that Rihanna song all afternoon.
The lagoon city of Venice is famous for its canals, but it is also regularly swamped by floods and high tides. The highest level of flooding in recent history was in 1966 when waters rose by 1.8m.