Для открытия водомерки на озере Кайя нужно учебное ружье - ближайшее сдается напрокат у нипа Мерио немного восточнее Северного порта Кайи

to get knowledge for them u have to rent a practice musket from NPC Merio and then shoot them. He can be found at the trade manager east of north kaia ferry

I had the knowledge and I deleted it with the Elixir of Oblivion and now I can't get it back even with the rented pistol from Merio. Can anyone else confirm this? Same goes for Jellyfish.

You need to crouch (space) while having a matchlock equipped (you can rent one at Merio at North Kaia Pier), before you shoot it at the striders, to get the knowledge.

You cannot get their knowledge by killing them normally or shooting them while standing.