"I see the sea."
Translation:Я вижу море.
There should be a subject in every sentence in standard Russian languge. If you do not put there я they will understand you but it is not correct.
i thought in russian you could say for example znayu or dumayu to mean i know or i think
Yes, we can say this way. But it would not be a translation of this sentence from the task, which has "я"
It is understood that the given translation has я in it, the question is why is the omission of я marked incorrect here?
Because the sentence from the task has "I" and you have to translate it to Russian. But try to report your choice.
"Смотреть" is "to watch," or "to look at" when used with the preposition "на."
Indeed this sentence does require the accusative case. Морю is actually the dative case of море. You can see all the declensions of this word here https://en.wiktionary.org/wiki/%D0%BC%D0%BE%D1%80%D0%B5#Russian
For the word море accusative and locative are the same but in this case it is supposed to be an accusative form.
it is already accusative, but because море is neuter, it doesn't change: http://www.russianforfree.com/grammar-of-russian-nouns-accusative.php