I've been given the 'info on the storm that wrecked your ship' quest but it turns out theres no way I can kill the Daedra in the dungeon I've been sent to... I'm going to have to fail it.

Will that mess up my game? Is it a vital part of the overall quest?

Nope, that one's perfectly optional. Heck, it may even be a random, reoccurring one!

I can't remember, do you have to kill any daedra or are they just guarding the scroll? You may be able to sneak past them, but that's unlikely.

On that quest the Daedra spawns on the scroll. So if the monster is not stucked in a furniture, you can lure him to some other part of the dungeon then run back to get the scroll.
If the daedra is stucked and can't move, good luck! (Well if you have a silver bow with silver arrows you can shoot it safely)

That quest is indeed random. Once, I did this quest three times in a single game. Seems like many people know about the player's shipwreck event.

That quest can be messy though. I once did a full search of the whole dungeon but never found the Daedra or the letter. So I used the cheat to teleport myself to the letter. Turns out the Daedra and the letter had spawn INSIDE a statue. No wonder I never found the damn thing.

It's a very very hard quest to get if you are low level since Daedroths and Daedra Seducers are the two you encounter.

It's a Daedroth and I can't even touch it with my elven longsword

Thankyou both for replies