I have been using water method for hard wood cutting for the last few years and the success rate in general has been pretty good but takes time and need to be patience. 
My question is can I plant my 2 years old fig tree in ground where the drainage is not good. That's why I pot all my fig trees in containers as I am afraid they will be killed by overwater and rot etc. (the drainage of my back yard is awful during winter and raining reason) Since water cutting is workable to produce roots and leaves etc., what do you think to plant in ground where can be overwatered for a few months in a year.

You have not bothered to show your location/zone in your profile. So as a general comment, you can plant in that location by making a raised bed (about 4'x4'). That will help the roots mass avoid drowning. It all depends on how bad is the drainage and the amount of rain but surely a raised bed can help a long way.

Sure many people grow in pots in BC and you know your ground drainage situation.
I lost two apricot trees in area that used to stay moist constantly, not too wet but moist.
All my figs are in pots.

I live at same river as you with same wet soils, I build 12 inches high raising flower bed with fig tree, Japanese maple , spring bulbs are all fine, Last year fall, I planted 4 testing fig trees without raising bed and 2 of them was having roots problem, so I build 2 small raising flower bed for them hoping that I can save them, Also, I use sand base top soils mix with mushroom manure and fir bark mulch, I did water test and its shows very good results to keep roots dry, Fig trees do not like wind,  I have strong wind problem as well, anyway, Keep trying.

Personally I think the best way to grow a fig tree in ground in my yard is to grow it with its pot and allow the roots to feed
from drainage holes. Most times I never bother to dig a hole. If I did, its not more than 2" deep. Note, I plug the drain hole
in the middle of the pot(if there is hole).

Does it make sense if I still place the fig tree in pot on top of the garden soil (raised by 5 - 6" height) and cut out the base of the container so that the roots an grow like in ground ? I did ask some local nurseries for same and the answers are 50% and 50%.

That will almost be like self-watering pots.

Fig tree roots spread out & stretch far. Once the roots start feeding out from the pot, the fig tree can take
off like a weed. Hence I like to root prune to manage tree size. This has never given me any less figs. I have 
trees with root rot when they they are less than a yr old but never experience root rot with older trees either
in pot culture or in-ground.

You may try to open up a small corner at bottom of your pot to limited roots spread out, you may test if for a year, If it work well, you can open up more bottom area of your pot for another year.

Basicly, roots inside pot provide water, air, nutrients, and roots in ground provides water, air, and nutrients, During wet winter months, If fig trees are still active, Its roots are still able to breathing air inside pot.