Peter's desire to gain access to the Baltic wasn't just because he wanted to build a nice big fleet (although he did want that), it was driven by his overall desire to open Russia up to Western Europe by promoting trade and cultural inflow. At the start of his reign, the only port available for trade with Russia was in Arkhangelsk, a northern port that was frozen over much of the year. In order to open Russia up to Europe, Peter needed to build a warm water port that wasn't frozen as often his only option was the Baltic.