I've been trying to go over every way this could've been an accident. Here's one way:
It's relatively late and Elisa grows bored. She's unable to sleep. The TV selection (I imagine) is extremely limited; she's in a city where she doesn't know anyone; and even though it's not really "late-late," it's still too late to roam the city for a tourist girl who knows next to nothing about her surroundings. So she begins to explore the hotel, going to random floors and walking around (I've actually done this before with a friend). At first, perhaps she decides to take the stairs, roaming up from floor to floor. When she's captured on the elevator footage, perhaps she was getting tired and wanted to start making her way back toward her room. She pushes a bunch of random buttons, still wanting to look on floors on her way back to her room (just to see how they are; what they look like; if they're nicer than her floor). Perhaps she was just going to peer out, keeping one foot in the elevator (in order not to lose it), and riding the elevator (eventually) to her room on the 4th floor. The elevator doesn't move, so she plays games with herself, jumping out, yelling "Ha!" laughing at herself and the fact that she can’t operate the elevator.
Eventually, she gives up on the elevator and takes the stairs. Suddenly, she sees the flight going up (could she see the door to the roof from the 14th floor?) and thinks it would be neat to go on the roof. When she gets there (via the alarmed door), she wants to get on the highest point of the roof, which is the roof of the storage room. She climbs the red ladder up. She looks out at the city. She looks down and sees the tanks below, wondering what's in them.
Here is how easy it would've been for her to lower herself to the tank:
http://farm9.staticflickr.com/8376/8491046418_037307c15e_o.png
Scenario A: She opens the hatch. It's pitch black. She isn't exactly sure what's in it. She reaches in, but can't feel anything (it was 3/4th's full, right?). She puts her head inside and leans in, when she accidentally falls in.
Scenario B: She realizes it's water, and decides to jump in with all of her clothes on; OR to strip (skinny dip thrill on the roof?), dropping them inside the open lid, before jumping in herself. She thinks it will be cool to swim in the tank on the roof -- wanting to experience something. She treads the water for a bit, then tries to exit when she realizes that she couldn't get herself out as easily as she originally assumed before entering. She starts to panic, realizes her clothes (if she's still wearing them) are weighing her down, so she hastily strips in the tank in order to try to jump up and pull herself back through the lid.
Scenario A is more far-fetched, in that she would’ve been bound to scratch herself somehow if she accidentally fell in. Scenario B is perplexing because why would she drop all her clothes inside the tank and/or enter the tank fully clothed, when it would make more sense for her to simply leave her clothes outside? The only answer would be that she wasn’t thinking at the time – a weak explanation, but one that definitively can’t be ruled out.