Use Butyl Electrical Tape. You can usually find it at any hardware store.
It is some bizarre stuff.
Here is a link that shows it.
http://www.alibaba.com/product-gs/511191869/High_voltage_electrical_butyl_rub...Note: It only sticks to itself.
Wrap the wires that are showing conductor with regular vinyl tape.
Wrap the whole mess with butyl tape. Stretch it as you wrap it around the bundle. Do about an extra inch or so on each end.
It will create a water tight seal. I love that stuff.
It's also known as "self-amalgamating" tape.. and probably other things as well.