Learn Arabic (Modern Standard): سبح الولد الرياضي في البحر.

This is the standard word order for verbal sentences in Arabic: Verb, then Subject, then the rest of the sentence. So instead of saying 'The boy swam', Arabic says 'Swam the boy'.

Why does الرياضي (athletic) have ال (the) attached to it?

In Arabic, adjectives come after the noun they describe and must exactly match it in being definite or indefinite. Because الولد (the boy) is a specific noun starting with ال, the adjective describing him must also start with ال. Literally, it reads as 'swam the boy the athletic'.

The word الرياضي starts with ال, but why is the 'L' sound missing when spoken?

The Arabic letter Ra (ر) is what we call a 'sun letter'. When a word starts with a sun letter, the 'L' sound in ال disappears and merges into the first letter of the word, creating a double 'R' sound. So it is pronounced 'ar-riyaadiyy', not 'al-riyaadiyy'.

If the sentence was about an athletic girl instead of a boy, would the verb سبح change?

Yes, the verb always has to match the gender of the subject, even when the verb comes first in the sentence. For a girl, you would add the feminine past-tense ending to make it سبحت (sabahat).