They get booted out of the trial... or!

If you really want to torment your character, the siren is an actual siren who is being paid to help, so you can totally say they decided they like your character and therefore drag them deep down through a secret tunnel at the bottom of the pool and try to keep them as a cute lil pet-snack. Iyaa, you're just so cute you corrupt little human~ Cheek pinching and feeding little kelp snacks and delicious fishies (though never cooked exactly right- they don't really get why humans need to cook their food, but whatever) and keeping in a little coral cage in a cave (above water, but you have to go through the water to get to it) and all that with the intent to eat once you've been plumped up a little! So you can have a second chance for rescue and/or if you really want your character to die (since that's sure an option) you can go for it. 'u' If your friends ask around the water's edge, selkies and some undines are willing to tell you how to get to that particular siren's cave.

Seriously, have fun with whichever sounds like you want to play with it.