This is an odd question, but why don't sims stay on their floating rafts (in the pool) for more than a few seconds? Not even if you push them to do so? I find this happens a lot when you tell them to sit too, even with autonomy off!  (I have only on rare occasions gotten sims to stay seated, but it's usually with autonomy completely off and having them chat seated, then they do stay seated for a bit longer even after chatting.  Do one of the mods give us an option to lengthen the time these actions last?  It's sort of the same with relaxing in the lounge chairs, but at least they do it for a little longer than a few seconds before switching chairs or gettin up.  This has bugged me for some time, I mean what's' the point if they literally only float there for seconds, even by sim-time standards that's quick.  I was wondering if maybe retuner or one of the other mods refrences this, but I didn't see anything. To be fair there's just tons to file through. 

Comfort could be one of the reasons sims will go out of their way to find the most comfortable object to sit on. But not always one of my sims absolutely refuses to use the Gothique Living Chair despite it having a very high comfort level so it could also be if they like the object since I notice she always has a thought bubble that shows the chair with a line through it and for some unknown reason will always use The Avant which has next to no comfort level but when I first bought it for her she gave the expression sims give when they love an object.
Another could be their energy sitting replenishes their energy to an extent once it reaches a certain point they want to move around again.
 

I've had them not sit regardless of energy level. I push them to sit and they stand back up after only a few seconds, every time. I also only use the nicer stuff so they should like it fine, no negative thought bubbles, but they just wont remain sitting even with autonomy off. I have also noticed that they wont keep watching TV if their fun need is full, but only when autonomous, I can usually get them to stay seated if they are doing something like TV or books or chatting, but to just sit?... apparently they don't care for this. LOL.

I've never had them stay on the floaty raft thing in the pool though, even when they do it autonomously they only float for a second before getting back off. IDK. no biggie, just wondered if it was just some routing/coding thing and if one of the mods would work to make them want to float longer. I used to sunbathe on those things and even doze off bobbing on the water. Got burned to a crisp that way once too! and I don't often burn easily either! :-o

Well see, then the solution is to have them nap on the inflatable rafts until they literally catch on fire.  It's fine—they can just roll off the rafts and right into the water.  But napping should work fine if it's enabled.

I can't get my sims to sit still, either.  It doesn't matter where they sit; if they're not doing something else, they'll get up almost right away.  I chalk it up to sim-ADD.  They also will stop watching TV or doing any other fun-only activity a few minutes after their fun bars max out, but only if there's something else tugging at them, like another low need or a screaming toddler.  Or a baby lying outside on the lawn.  Of course I don't do that.  Anymore.

Well sims are picky and finicky. Just because an object is nice doesn't mean they will like it. I used the Gothique Living Chair as an example it's one of the nicest objects and The Avant is one of the cheapest. She gave the happy dance when she got The Avant and cried when she got the Gothique Living Chair the negative thought bubble tells me she absolutely hates it her son loves it though.
Her son I can't get him to stay and watch TV unless it's the cheap Old Timey and he absolutely refuses to stay put when watching the WallVu.
I could understand it if they had the Frugal trait but they don't. I can only chalk it up to sims are just strange.

Sittingbear - I played last night and it was time for a birthday party, so I turned off autonomy and brought everyone to the table and dining area to sit down while dad/son blew out the candles. To my astonishment all but 2 of the 10 stayed seated until it was time to scream and blow the whistles. I swear that was the first time in a year of playing I got so many to stay put while everyone in the house made their way to the table! As I stated they usually get up and walk a couple of steps away and all stand staring into different directions. but what was even more odd was that a few of them chatted while sitting and that was with autonomy off! I was good with that though. 

Your sim will sit forever on the pool lounge if you have books in their inventory. Once they lounge, click on a book and cue as many books as you want. Just make sure they don't get sunburned!