Ahhh it's because the Obsidian Blade is an ice element, and Teleri is strong against ice. (The Poison spell also has a 50/50 and will be affected by the weapon you use, but I think I'll set it at next update to just hit 100% of the time instead. Ditto with being clearer on which weapons have what elements)
Choosing to go into the waters with them will always get this result (either ondins or naiads depending on your gender preference) so the only way to avoid it is choose not to go into the water