It seems like both ボウル and ボール are valid Japanese words. The general impression I get is that using long vowel marks ("ー") are the norm for words written in katakana.
In Why are long vowels represented differently in hiragana and katakana? , explanations of vowel kanas being used instead include words that have kanji, or breaking up the sound. Maybe I'm just ignorant of how "bowl" is pronounced in either British English or American English, but neither explanation seems plausible. Why is ボウル spelt using a vowel kana?