It's free nitrogen!

A number of factors here! Chlorine and saltwater are huge contributors, and swimming tends to involve a lot of sun exposure, plus there's tangling and agitation.

More specifically, agitation of any animal hair fiber while wet means the cuticle scales have been softened by the water, so they've lifted up a little, and each strand is rubbing with its neighbors, scraping and damaging each other's lifted scales.

Incidentally, this mechanical agitation of strands against one another while they have lifted and softened cuticle scales is also how woolen clothing ends up felted if you throw it in the washing machine.

Water sports is what she does to your compost.

I was so certain!!

Nothing can actually repair a damaged hair strand, since they are non-living cells once they leave the follicle, so the only actual fix once the cuticle is physically damaged is a trim with very sharp scissors. However, any product that coats the hair shaft in molecules that will fill in a little of that damaged surface and provide slip will help somewhat with tangling and visible dullness, such as repairing and deep conditioning hair treatments that contain lipids (oils and fats) and/or silicones (dimethicone, trimethicone, dimethiconol, etc.). Silicones can also temporarily sort of glue very small splits together until the next wash, although a split like this one is far beyond their powers.

The down side to silicones is that the way they coat the hair shaft can also keep moisture and lipids out, leading to dryer hair over time, so if you go that route, it's a good idea to use a harsher detergent like a clarifying shampoo once a week or so, followed by some kind of silicone-free lipid treatment, like a shea butter leave-in.

Oils and fats are great for prevention and conditioning! Sebum is a hair conditioner that your body manufactures, and any lipids with similar properties are helpful. You can apply them straight up as an overnight treatment, or use them as a light daily control and protection product, such as by rubbing a little almond oil or shea butter between your palms and finger-combing it through damp hair, although heads up that with hair as fine and straight as OP's it's easy to accidentally use too much and end up looking a bit greasy. Curly, kinky, and thick hair takes really well to heavier use of oils and fats; this is why the Black hair care aisle is shea butter heaven!

For immediate care after a color treatment that incorporates a lifting agent, especially lightening treatments, a protein treatment product like KeraFix can lead to better long-term hair quality by replacing some of the lost protein molecules and keeping the weakened hair from stretching as easily, which does a ton of damage.

And I'll stop there, since this comment is already very long.

Yes! It's free nitrogen! Everyone should be peeing in their compost! Unless you're taking certain kinds of medications, in which case, maybe don't add e.g. a bunch of lithium to your vegetable garden.