For your ranger what are your Dex and Str scores and what kind of weapon are you using? You should have a +4 or +5 to your attack mod, which should give you higher than 50% for the imps on the ship.
Also there are only 2/3 mandatory fights on the ship: 1)Imps after meeting Lae'zel. 2)Imps at the top of the stairs (possibly skippable). 3)Assorted devils/fiends at the head of the ship, not including the cambion or mind flayer.
Any additional fight is likely to be more difficult, but still doable.
