I hear a lot about battles between armies and sieges during the middle ages. But I've never heard of a naval battle. So I'm wondering if it's just not an area that receives attention, or if naval warfare during this period was just so rare that it's not mentioned.
Depends on the specific area and time period. I've only read about Norway, which is more geographically geared toward seaborne travel and fighting, but I'll throw it in.
Several key battles in Norwegian history occurred at sea:
The battle of the Hafrsfjord (872), where modern West/Middle Norway was gathered under one ruler for the first time.
The battle of Svolder (1000), which led to the death of Olav Trygvasson and another cycle of puppet rule under Denmark.
Several key battles of the Civil War Period (1130-1340) occurred at sea, with five pretenders to the throne being defeated and presumably killed at sea.
Western-Northern Norway's peculiar geography (deep fjords, tens of thousands of islands, rough mountainous terrain) made maritime travel the prime mode of transport for powerful people wanting to assert and maintain their dominance over mainly coastal power centres. Thus Norway had a much stronger maritime military tradition than places like Germany and France, where marching a large army over land was far more practical.
Source: History of Norway 800-1536 by Jón Vidar Sigurdsson