Easy.. Saltwater Croc
The Tiger shark will be swimming around the pool because they're always moving. Even if it doesn't get you you're going to shit yourself every time it heads towards you.
The Saltie will just sit at the bottom. I'm assuming you can chose the corner you get into. Just hop in the furthest one away that he's not looking at and prey he doesn't notice.
If he does then get the fuck out quickly.
