Was ist braun und schwimmt unter Wasser? Ein U-Brot.

What is brown and swims underwater? A sub-bread.

This is a wordplay joke. 'U-Brot' in German sounds like 'U-Boot' which translates to 'submarine' in English. Therefore, the joke is switching 'U-Boot' (submarine) with 'U-Brot' (sub-bread), creating a fun and unexpected twist.