Re: [BY DESIGN] [PPSP] Sims will not stay in the hot
I believe this issue as being described is by design. It's true that in Sims 3 Sims could sit in the hot tub for hours, but that's not how it's ever worked for TS4.
The Get In interaction will leave the Sim in the chosen interaction until their Fun motive is full. Once their Fun motive is fulfilled via the user directed Get In, they are open to selecting a new interaction. Given that the Hot Tub is only a Fun value of 1, it is usually outscored by other objects. And if all motives are satisfied, the Sim will seek out something new for Fun.
In my own game, my Sim had maxed Fun and stayed in the hot tub for less than 1 Sim hour before getting out to play on the computer. If I tanked Fun, my Sim stayed in the hot tub for about 8 Sim hours before choosing a new action.