I had an experience like this up near Sucia Island a couple years ago. I had about 200 ft of downrigger out with a flasher and hoochie in about 400 ft of water. The take down was so violent I was sure I had hooked something on the bottom. Absolutely spooled everything. I had a pretty good sonar at the time and it showed nothing by the time I could get my wits about me. Went back to take another pass just to see if I missed an underwater obstruction, but none was found. I figured a seal or ????