I had an "issue" with one last fall. It was the only time I've had a problem but it lasted for several days until the thing migrated away finally.
First time I saw it I was pre fishing and it was swimming straight toward me. Really didn't think anything of it at the time as I was concentrating on the fish but it must have slipped under the water without me noticing. When I got my lure back to the boat the dang thing shot out like a torpedo out from under the boat and at my jig. It disappeared, then did it again on the next cast. The crazy dang thing was using my boat as cover! Every time I got the lure close to the boat the loon would pop out from under the boat and give chase. I think he could have easily grabbed my lure, especially on the first couple casts when I wasn't prepared. But I believe he was waiting for me to reel in a fish or for one to follow my lure in so he could snag it.
I tried smacking the water with my landing net and running from him for a while with my TM but the bird wouldn't leave. He was dang persistent. I finally had to fire up the big motor and leave.
On the mourning of the tournament I show up at my first spot of the day. I catch a nice smally on the first couple casts. But the noise of the catch attracts the attention of, you guessed it, the same crazy loon (he had unique markings so I know it was the same bird). He swam right over to my boat and dove under it to use it for cover from which to attack. This spot was about a mile or so from the place I saw him the prior day. I had to give the spot up because I didn't want to waste tourney time messing with him. Figured as long as he was around the bass would be leery to say the least. That dang bird sat on that spot all morning too. Really fried my game plan for the tourney since that was the only place I had found decent fish in practice. Aghhh!