Adding lines is a good solution, but I have suggestions:
1. You can lighten the lines at the moment of contact with water.
2. You can make colored lines against the background of the skin. (e.g. Kana's palm overlapping Kasumi; Yui's hands)
3. You can try to trace Kana’s pupils (situational sentence)
I can’t say exactly how much better these suggestions will be, but this is what came to my mind, if you want, try it.
And now about the important thing: It looks soooooo soft!!!!! You did a good job on these lines and my eyes love it!