Please help

I read this manga not too long ago but I can't seem to find it, it's about a guy who goes fishing with a group of people and when they fishing a big black shark come to them and I think I this young the guy is in the water and so the there's another shark I think trying to attack him but the first big black hat protects him and then I dont know how he ends up on his fin and then they washed to shore and the shark turns in to a man and then they have sex ......