Uida

It's time to go swimming! The four important forms of this verb are uida, uin, uin and uinut. 

• Osaatko uida? - Do you know how to swim?
• Mennään uimaan! - Let's go to swim! 
• Mä uin eilen tuon saaren ympäri. - I swam around that island yesterday.
• Mä en ole koskaan uinut meressä. - I've never swum in a sea.
• Hai! Ui! - A shark! Swim!

When you swim in a lake or in a sea for the first time after winter, it's called talviturkin heittäminen - throwing away the winter fur coat. 

• Oletko jo heittänyt talviturkin? - Have you been swimming in a lake yet?
• Minä heitin talviturkin jo maaliskuussa! - I swam for the first time already in March!

Here are some sentences that you might need on the beach:

• Onko vesi kylmää? - Is the water cold?
• Haluatko lainata mun pyyhettä? - Do you want to borrow my towel?
• Onko sinulla aurinkosuojavoidetta? - Do you have sunscreen lotion?
• Otitko eväitä mukaan? - Did you bring any snacks?

Some time ago when the sexual harassment law was updated, they added a special sentence to make sure that the Finns don't have to change their summer cabin swimming habits. (Feel free to correct my translation as I'm obviously not a law expert.)

• Suomen oloissa ei esimerkiksi sitä, että ohikulkija näkee kesämökin rannassa alastomia uimareita, voi pitää tämän seksuaalista itsemääräysoikeutta loukkaavana.  - In Finland, you cannot consider it to violate your right to sexual autonomy, if you see naked swimmers by the waterfront of someone's summer cottage.