"We see the sea from the hotel."
Translation:Otelden biz denizi görüyoruz.
In this case "bir denizi" means "a sea", since the presence of "bir (one, a/an)" indicates an indefinite noun. I'll try to explain it to you with some examples:
deniz > NOMINATIVE, simply "sea" or "the sea (definite noun)" / e.g.: "Deniz suyu" > "seawater" or "Denizi görüyoruz." > "We see the sea."
bir deniz > NOMINATIVE, a sea (indefinite noun) / e.g. : "Orada bir deniz bulunuyor." > "A sea is located there."
denizi > ACCUSATIVE form of the simple noun "deniz" or the definite noun "deniz" / e.g.: "Denizi sevmem." > "I don't like seas/the sea (A sentence like this in English has to use the noun with the definite article or in the plural form; however in Turkish, the noun can be simply used without an article.)." or "Denizi görüyoruz." > "We see the sea."
bir deniz > ACCUSATIVE form of the indefinite noun "bir deniz" / e.g.: "Bir deniz gördük." > "We saw a sea."
bir denizi > also the ACCUSATIVE form of the indefinite noun "bir deniz"; it doesn't differ from the accusative "bir deniz" in the meaning so much and it has a rare use, it is usually preferred to make an emphasis on the word. / e.g.: "Bir denizi sabah görmek istiyorsanız, erken kalkın." > "If you want to see a sea in the morning, get up early."
Turkish noun cases can be a little confusing from time to time, but the best way to learn them is creating your own sample sentences with them. I hope you find this helpful, I wish you plenty of success in your course!