You really want an answer to this problem? Okay, first off, I actually HAVE experience doing this kind of thing. Not to this extreme, but B&E, Oh yeah. So, here we go. We're taking the water route.
Piranah are no problem. Contrary to popular belief, they're scavangers, not predators. Unless something is wounded and bleeding, they're not going to bother it. The electric fence isn't going to be a danger, since it's in the water, which means it's utterly shorted as soon as it was turned on(and I know it's in the water because you said the fish are on both sides). Even if it was on a wall above the water though, using a thrown cable or rope I could short it against the water. Fence is done. THe lock is a lock, locks can be subverted or destroyed, my choice at the moment. Snipers aren't that big a deal. Underwater using a snorkel I'm practically invisable. Even if they did see me though, the water would alter the trajectories of the bullets or destroy them within the first foot or so, so they aren't a threat. After I approach the building, rather than mess with the guards, I'm going to scale the side of it until I reach the floor I need. Then I'll use gas bombs(smoke or nerve, preferably nerve) to clear out any guards. Grab the package, dive into the water and return.
Edit: Oh, and for reference, a list of necessary equipment.
Climbing spikes(Hands)
Climbing spikes(Feet)
Bolo/Quantity 1-3/3 ft. length/metal weighted ends
Explosive gas bombs/powdered incindiary neurotoxin
Throwing stars
Caltrops
Snorkel
Plastic Bag(for package)