I make hemp bracelets, but the only problem is that any dye in the hemp comes out when they get repeatedly wet, and they also stretch and fall apart after so long.

What can I do, other than take them off before every shower, to save them?
Maybe I can waterproof them?
Somehow?

You could try a thin varnish coat after the jewellery is created, but this might harden it and it will add a sheen to the jewellery.

Or you could try beeswax.
Beeswax Uses