Question about taking my pet on a long ferry crossing?

Hi, i am due to take my dog (yorkshire terrier) on a 20 hour ferry crossing from Santander to Plymouth. Some people have told me about giving her a tranquiliser to make her sleepy but I am a little worried. I have tried talking to the vets here in Spain but as things are different here and their English is not very good and my Spanish is not very good its hard to know for sure. An English person I spoke to said they done it and gave their dog human travel sickness tablets but I am worried. Does anyone have any advice or have they experienced this before? thanks