May I tovel somonene elses keilim if they allowed me to use them but do not know about tevillah?
Is there a diffefrence with a keli that may break like a kettle, or tovbeling in the sea?
If the person allowed you to use the kli, they would want you to do tevila on it, and use it properly, especially if they don’t know that it has to be toveled. The reason we do not tovel something of someone else is because they may not want us to take their mitzva away from them, however if the person doesn’t even know about the mitzvah of toveling, he isn’t going to mind if you tovel it for him.
When toveling, you have to be extra careful that the item shouldn’t break, and if you break it you will have to pay for it. You can tovel keilim in the sea.