Dap in the tub?

We have a small crack in the tub that I want to repair.

I thought that Dap caulk would work, so I cleaned the tub and let it dry for 2 hours.

Then I applied the Dap and left it for almost 12 hours even though the bottle says it only needed 2 hours.
I took a shower earlier this morning and it was fine.

I let my daughter take a bath just now , and I see about 30 minutes into her bath that the Dap has begun to wash away and 'pill up'


So, what can I use to seal this hole that is water safe?