I've been to Liverpool many times as a foreigner and never had any problems understanding Scousers. Of course there's a difference between a cab driver, bartender and hotel personnel for example, but they're all capable of speaking perfectly good and understandable English if they want to. And even if they use their Scouse accent, it's still not very hard to understand them if you listen carefully. It might be an awful accent if you're used to hearing proper British every day, but I don't mind it at all tbh.
Is there really such a difference between their accent and the one used in Manchester, considering it's only 30 miles apart or something?