I speak Italian at a decent level (though worse than English) and was able to communicate without problems when I was on Barcelona (Catalan) and Madrid (Spanish). I talked in Italian with a weird accent, they replied to me on Catalan/Spanish and it worked perfectly. If really concentrated, I can also understand a bit of Portuguese (though no French).English and German are both Germanic languages. That doesn't mean I can understand Das Boot without subtitles. 'Most can understand Italian straight off the boat' seems unlikely in the extreme.
For natives, it should be significantly easier.