"I sometimes go to the sea by myself."
Pronunciation of 一人 is more common to be "hitori". It's better not to be taken apart.
Agreed, I think it would make sense to have the two kanji together so that you hear the correct pronunciation when you click on it. That being said, the amount of different counters in Japanese blows my mind!
I'm thrilled: I just knew that "umi" means "sea" because of my childhood days when I was playing Yu-Gi-Oh. :O
i'm gonna thrill u some more: umineko ( seagull in english) means a sea cat. isn't it great?
In this case で indicates that action is performed at or by the noun before で. 一人で- by one person(here interpreted as "by myself) , 一人- one person