The other day, my sim and his kid found themselves at the Harbor Quarter Gym with the rest of the household so I had the sim "coach swimming" for his son.
It didn't raise the Motor bar any faster than, say, playing Keyboard Commander. It took roughly an hour to raise the Motor by one-third.
However, when the son aged up to a teen, I happened to check his Bodybuilder aspiration and he already had 4 out of the 8 workout hours completed for the first tier.
It looked like this:
Work Out for 8 Total Hours: 4/8
Since the son had never worked out since his very recent teen birthday, I'm wondering if "coaching swimming" later translated into Fitness levels.
Edited to add:
Results of Testing Using 3 New Sims
Child A (Geek) and Child B (Loves outdoors) both continued swimming for a period of about 6 hours, although all three (father + 2 sons) got interrupted frequently when six sims later arrived at the pool (de Krill or something) to dive, splash, swim, and try initiating convos.
The father had zero fitness while first coaching Sim A for the first half. Next, I cheated to give him Fitness L10 and he coached Sim B for the second half.
All three went to a restaurant so the father could order a birthday cake (since he can't cook). The teens' Bodybuilder aspirations looked like this:
Teen A Work Out for a Total 8 Hours 5/8
Teen B Work Out for a Total 8 Hours 6/8
The hour difference is only the result of interruptions by other sims; each child sim was either swimming on his own or swimming while being coached.
The coaching sim's fitness level made no difference at all. The Motor bar rose at about the same speed, regardless of being coached or not.
And now the question is: What's the point of coaching swimming? (besides the fact that it's very cute to watch)