I have had the same problem. My doctor tells me that age and hormone shift as I'm getting older causes a decline in hair and nail health. Also, low iron. She recommended taking a Biotin supplement. I did that for 8 months with no improvement. Haven't tried collagen yet. As a means of prevention, I clip my nails down before a dive trip. They can't break if they're not there to start with.
There are a few oils you can use post dive for moisturizing the body, hair, and nails: argan, vitamin E, coconut, frangipani monoi. My favorite is Frangipani Monoi Oil by Elemis. It absorbs quickly and doesn't leave you feeling greasy, and it smells wonderful! In its natural state, it is in solid form in the bottle. In a tropical climate, it will stay in liquid form. At home, I fill the bathroom sink with hot water and let the bottle sit immersed, and it's ready to use after a shower.