Jamaica Joleen,
I never heard of a northern attack but there have been documented musky-on-swimmer attacks reported in the generally reliable WI Trails magazine.

If you think about it a musky (or a northern) bear a striking resemblance to baracuda - except I rthink muskies get beefier than baracuda.

If you haven't snorkeled Devils Lake it is definitly worth while, possibly the most beautiful location in WI.