In season 1 ep. 5 we meet Sesshomaru for the 1st time. In this ep. We see sesshomaru take a boat onto the water. Later in the anime we see him in the sky. If he could float or fly around in the sky then why did he need a boat?

That is a really good question! As what i figure, there might be three positive reasons, 1/ If Sesshomaru is flying in the sky all the time then it might seem boring for the audience. 2/ In the beginning of the show, Sesshomaru was on the boat with Jaken at a pitch dark and foggy night, he might not see a thing if he was flying in the sky. i guess boating is easier for him in that case. 3/ I believe that Sesshomaru could do anything that he wanted to do. i can see from the show that Sesshomaru chooses things with his will, and sometimes he would just go on in his own way.