Hey, good for you, Colors! Tznius is one of the most beautiful mitzvos we have, but it can be difficult at first, so it’s really inspiring that you’re starting the journey! As far as swimwear and active wear goes, there is a wide availability of modest attire. I would suggest checking out sites such as modli. The fully-covering dresses/shirts&skirts are made of bathing suit material so they’re super comfy and versatile, and they dry quickly (somewhat pricey).
Gila Manolson has a book on tznius called “Outside/Inside” that I would recommend.
Hatzlacha! Don’t be afraid to keep asking questions! We all do it 😉