Using hand-recorded shipping data from the Climatological Database for the World's Oceans, history graduate student Ben Schmidt mapped a century of ocean shipping, between 1750 and 1850. The above map animates a seasonal aggregate.
There aren't many truly seasonal events, but a few stand out. There are regular summer voyages from Scotland to Hudson's Bay, and from Holland up towards Spitsbergen, for example: both these appear as huge convoys moving in sync. (What were those about?) Trips around Cape Horn, on the other hand, are extremely rare in July and August. More interestingly, the winds in the Arabian sea seem to shift directions in November or so. I also really like the way this one brings across the conveyor belt nature of trade with the East.
The bobbing month label is distracting, but its position actually does mean something. Since seasonality (i.e. weather) plays a role in travels, the label represents noontime location of the sun in Africa. Okay, I'm still not sure if that's actually useful.
If you really must, you can also watch the century of individual shipments during a 12-minute video.
By the way, Schmidt used R to make this, relying heavily on the mapproj and ggplot2 packages. (Bet you didn't see that coming.) I think he created a bunch of images and then strung them together to make the animation.