Please dont kill yourself.
No seriously.... dont think about doing this yourself especially if you value your life. The cost of an electrician to do what you need is nothing compared to what it will cost you to fix what you do wrong when it goes wrong.
Do not even THINK about mixing water and electrical conduits, your just asking for trouble.
Electrical cables should be buried by regulation to 70cm underground, then covered in cement, about 15cm thick and then with plastic covering over the top in bright yellow stating that there is electricity cables beneath.
If its a garden light, the rules are more relaxed... But all the same, keep the tubing in one piece, use waterproof exterior plastic tubing, and make sure people know where the cables are in the future, so your gardener doesnt cut through them with a spade. AND EARTH EVERYTHING EXTERIOR.
The 70cm rule applies to heavy load cables, not garden lights, we are talking supplies to a shed, or exterior socket.
Feel free to post for more information but please consider that it wont cost you that much to get a professional to do it, just make sure that if you are not sure about what you are doing, get help from a pro.
Cheers