weird sea creature?


I'm not gonna put a lot of detail into this because I'll end up just going on some unrelated rant about the lego swamp creature.
But when I was fishing near the start of summer I glanced up at my screen and I noticed some weird green thing (that looks like the lego swamp creature) just in the water. It had it's head above the water and then after a few second just slowly swam away off the screen. I have no idea was and hoping someone here would. I was very confused and honestly slightly creeped out because I've never noticed this before.

I could just be seeing things but I wanna make sure. I can't seem to find it on the wiki.
Thank you for reading my tangent.