S (trusting the girl's): This part of the sentence establishes the subject's mindset. It shows that the person observing has a particular attitude or trust in the girl's abilities. It sets the stage for what follows.

P (swimming skills): This phrase is linked to "trusting the girl's" and specifies the skill in which the observer places trust. It clarifies that it's the girl's swimming skills that are being referred to.

R (he observed): This part introduces the action, which is the act of observation. It tells us what the subject is doing.

Q (the coming tide with an absorbed attention): This part describes the object of the observation and the manner in which it is observed. It tells us what the observer is watching (the coming tide) and how they are watching it (with an absorbed attention).