Bog Divers can be an extreme pain. Indirectly related to Fatties, Bog Divers are an aquatic ANMC and are the smaller brethren of the Sea Diver They have very good HP and they only pop up when they are going to attack. Their spit attacks can also be hard to judge and always when fighting them, Aya doesn't have a lot of space to maneuver.
That being said, they are weak to Fire so Firefly rounds, Combustion and Pyrokinesis can kill them almost instantly. Without Energy Shot, Firefly rounds can kill them in two shots, Combustion works (only if they are grouped up which rarely works) and Pyrokinesis works very well to boot. From what is noted, they will only pop up to attack Aya when she is standing still which is important to note.
Now they can be encountered in shallow water but this extremely, rare. Here they are extremely easy to defeat as they are aquatic, and can't do well in this environment.