Finding a cap to handle your amount of hair could be difficult.  You might find a latex type cap that is stretchy enough to pull over a bun.  Look where they sell caps for competitive swimmers - those are the most flexible.

When I swim, I try to protect my hair with oil or conditioner if swimming in open water.  I start by wetting my hair with fresh water, then apply oil or conditioner, and braid it.  When done swimming, rinse with fresh water.

When swimming in a pool, they usually don't appreciate the oils or conditioners in the water, but I will make sure my hair is at least rinsed with fresh water first, apply oil or a small amount of conditioner, braid my hair, then put the braid in a bun and cover that with a plastic bag, covered by a latex or spandex cap. (Spandex caps don't keep the water out, but it does keep the hair from coming loose.)  And of course, I rinse it all when done.

I have soooo much difficulty getting swim caps on my head, I can't imagine trying to get it over hair that was knee length.  I go with the fresh water rinse.