I've been doing this ever since I was a kid.
Decalling can be very stressful and very overwhelming to many. You spend countless hours honing your skills and building a kit you are "Hoover Dam" proud of. Everything has gone great or you have been able to correct any problem you have had. Then comes the dreaded decal laying. Sometimes they never get done and we make ourselves feel better by convincing ourselves the build look fine without them.
Or we put the kit aside with great intention to work up the courage to do them. Sometimes they never get done.
So you've gone and said to yourself "Hoover Dam it I am going to do this" , you get all set up. You put the decals in and you get on with it, then things start to go off the rails, you loose a decal in the water. The problems just snowball from there.
I started doing this. The sponge insures you never loose a decal in the water, and you can have many decals on deck soaking. It makes the whole expireince mush less stressful. I also use tiny paint brushes to manipulate the decals.
I fill the container just below the sponge.
Happy building.