First...In earlier Elder Scrolls if you had a compelling reason to kill an NPC and they were in a town, you could make them pick a fight with you. In Oblivion, casting was not a crime (one reason I liked Open spells) so it was possible to cast frenzy and then kill them in a fair fight. I'm not good a picking pockets or adept level locks, but I need that key. If I cast frenzy in Morthal am I going to get a bounty, arrested or have everyone hate me forever?
Second...I ran into my first slaughterfish but I couldn't kill them under water. They were near an island and I managed to lure them to the surface where I electrocuted them. I had to fight them to get to Treasure Chest #10 but nothing I tried while submerged seemed to have an effect. What didn't I think to try?