"A boy is swimming in the ocean."
Because "oyogimasu" means "swim" whereas "oyoideimasu" means "swimming"
In Chinese, 海洋 feels like an overarching category of things. For example, we say:海洋 can only be found on earth. In comparison, we say: I want to swim in the 海。Although these two terms can definitely be used interchangeably, we tend to treat 海 as more conversational whereas 海洋 slightly more formal.
In this lesson unit they have translate うみ as ocean and sea, is it correct? Dictionary gives 海 「うみ」for sea, and 海洋 「かいよお」 for ocean.
In colloquial, 海「うみ」means both, oceans and seas.
In the geographical term, we use 海「かい」for sea, and 洋「よう」for ocean.
- Mediterranean Sea (地中海「ちちゅうかい」)
- Caribean Sea (カリブ海「カリブかい」)
- Red Sea (紅海「こうかい」)
- Yellow Sea (黄海「こうかい」)
- East China Sea (東シナ海「ひがししなかい」)
- Pacific Ocean (太平洋「たいへいよう」)
- Atlantic Ocean (大西洋「たいせいよう」）
- Indian Ocean (インド洋「インドよう」)
- Arctic Ocean (北氷洋「ほっぴょうよう」or 北極海「ほっきょくかい」)
- Antarctic Ocean (南氷洋「なんぴょうよう」or 南極海「なんきょくかい」)
洋 (or 海洋 or 大洋) is just a very large 海. 海 designates a large area of salt water. So one will use 海 to designate an ocean when what matters is that it's a large area of salt water, and 海洋 only when it matters that it is one of the largest on the planet. So here, 海 is the way to go.
The verb I learned for swim is 泳ぐ（およぐ）, so what happens to the ぐ? How does it go from that to いで? I might have expected ぎ at least.