むろん、舟は何百艘も水面を動きまわっているが、どれでもいいというわけにはいかない。

、、。

Of course there are hundreds of boats moving around on the water, but not just any one will do.

Literal

Of-course, boats [topic-は] many-hundreds also water-surface [object-を] moving-around [but-が], which-one-also fine [quotative-という] way [to-に] not-go.

むろん ('of course, naturally') is a formal synonym of もちろん. 何百艘も ('hundreds of boats') uses the counter 艘 (そう) specifically for boats and ships. どれでもいいというわけにはいかない ('it's not the case that any will do') is a nuanced negation — ~わけにはいかない means 'one can't just do X.' The sentence sets up a selection problem amid apparent abundance.