We had a plaster crack like that that was leaking water. My PB repaired it with a two part putty that cures underwater. Mix it up and go mash it into the crack and smooth it. It comes in black and white that can be mixed to match grays. It hasn't leaked for 5 years or so.
Or what he ^^^ he said.
Like this
0530318 - PUTTY, EPOXYBOND - WHITE (SET) (530318)
www.inyopools.com