Answer:
When Captain Nemo says "Yes, I love it!" He is referring to his love for the sea. When Nemo also says "The sea is everything...." He is referring to the vast size and the part that the sea plays in his life. When he also says "There I am free" He is referring how when you sail into the sea, he feels free because there isn't anyone else to tell him what to do, therefor he can do what he wants.