The story
- French · borrowed frombête de la mer
- English todaybedlamer
Borrowed from French bête de la mer. The construction and spelling of the loanword was evidently influenced by Bedlam and bedlam, suggesting reanalysis as bedlam + -er. This seems all the more plausible given that these seals are sometimes quite vocal, doing a lot of barking, yelling, and mumbling.
Etymology from Wiktionary, CC BY-SA 4.0.