Hi Nicola, honestly, you'll be fine. The instructors are great and want you to pass and will do everything they can to make sure you know and understand everything.
The wet ditch drill firstly involves getting up at stupid o'clock in the morning and dragging yourself into the base while it's still dark! If I remember rightly, you get in and swim a length of the pool to prove you can actually swim. You get out of the pool, put on a life jacket and inflate it, get back in the pool and drag one of your colleagues to the other end of the pool, they drag you back. Get back out of the pool, deflate your life jacket and tie it up again, throw the life jacket into the middle of the pool. Get back in and swim to a life jacket, put it on and then blow it up using the mouthpiece at the top. Then they chuck a life raft into the pool and you all have to get in it. The first two lucky / unlucky into it end up hauling everybody else in. That's probably the hardest part of it all!
Then back to the base, cold and with wet hair for another day of learning.
Hope that helps x