My question is regarding the usage of IPA. I have seen some of the people submerging their printed objects in IPA before UV curing it and there are some who do not use IPA at all. So, why we need IPA at the first place? What are problems will occur if we do not use IPA for post processing? Is there any alternate method for UV curing if we don’t have those specific lamps etc?
Once your parts are done printing, they will be covered with uncured resin. You need to clean it off with some kind of solvent - IPA is the best option I have found. I use a sonicare toothbrush to clean my parts off - it does a good job. I also have an ultrasonic cleaner that I use for parts with inaccessible features (though I still clean in the IPA tank to remove most of the uncured resin first).
For UV curing I just bought the CUREbox from Wicked Engineering, but I have been using a nail curing station I got off Amazon for something like $20. Worked fine, though it is worth exploring curing in a water tank. You can alternatively set your parts in the sunlight as it provides UV though you will have to play around with cure time etc.
IPA will degrade the part especially if the part has thin features and/or is submerged too long. Warping, brittleness is a problem with IPA and with some 3rd party resins it will craze the surface (look like a desert).
It really also depends on the part your printing. With a water based system I have found the parts are much more durable and do not have the warping and cracking.
I have been using one of the plastic containers supplied with the printer filled with plain water and I hang a Reptisun 10.0 UVB bulb over it about 6" away for 5-30 min depending on part size and thickness. Cost is approx. $30.00 USD + the water 
You can read this thread Replacing Isopropyl Alcohol with Yellow Magic
Z-vat sells YM and Amazon as well.
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Has anyone cured the castable resin under water?
Should work with all the resins. Water inhibits oxygen from oxidizing the curing process.
Soaking parts in IPA after printing is a necessary part of post-processing. As @themedulla mentioned, there will be excess resin as your parts once they come out of the printer and this is best dissolved off using IPA.
After excess resin has been removed, there are a few different options for post-curing. We recently announced Form Cure which automates this process for a reliable and consistent finish but there are other options like using a UV nail salon lamp that may work.
I have been able to remove all uncured resin off the parts without the use of IPA. Heating YM will decrease the runtime needed in an ultrasonic cleaner. Running at aprox 120-140F will do a great job.
Using IPA i have been unable to make a direct mold of the part without the part shattering in my vulcanizer and the parts tend to be distorted especially ones that are thin and flat like coins.
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The Castable V2 Resin has a tendency to precipitate white residue during long prints which are attached to the part after the print. They can be washed away with IPA after the print.
Post processing with Water, even in an ultrasonic bath, won’t wash away the residue.
Does YM work better to make parts less brittle? I’ve found that the regular resins can break pretty easily, I do soak of about 10 minutes in IPA before washing off, if YM does better then I’d definitely switch.
Also remember people–don’t us IPA in a sonic cleaner, it can be a fire hazard.
I use primarily clear resin since I am only using them as master patterns for mold making from rtv silicones to organic rubber for spin casting and as patterns for delft clay (similar to sand casting).
I have not tried it on castable resin.
Initial wiping of a part in IPA or quick dip won’t make a noticeable difference but the prolonged soak is where I found the parts will degrade rapidly because IPA will penetrate the model. On 3rd party resins I actually got crazing where the surface looked like a desert floor.
At room temperature YM Does fine and will clear off most parts. Heating it really makes It work much better. I would not heat any plastics more than 140°F and most will really soften at 180°F. So far between room temp and 120°F seems to be good. Most water based cleansers may cloud when heated too much but this is only cosmetic and they usually return to their initial state.
I have not found YM to work especially well. I use it full strength for the initial cleaning out of the printer in an ultrasonic. Since I bought it, I use it. I cuts down on the volume of IPA which I now only use with a brush to get the best cleaning possible.
BTW… perfectly safe to use IPA in ultrasonic if sealed in another container.