This is what I did when my children were not co-operating to leave the pool ...

1. if there are peers, sometimes, they leave when they see their friends leaving

2. Priming, Priming, Priming. Have a visual + script.
"ok we go to the pool, at 2 pm I will start the timer, to give you 15 mins warning. I will give you 3 more warnings before it's time to leave the pool" .

Along with this script, also give your child accommodation - "ok if you want to stay longer, I can give you 5 more minutes but you have to use your "insert whatever mode of communication your child use to communicate" when you use "words" (in this case the mode of communication - you get to extra 10 mins when we go to the pool"

or you can say ... ok I can give you extra 10 mins but after that no more, we have to leave.

3. Set expected behaviors ... when the timer beeps ... you leave the pool, take the towel, dry your self etc .

Sometimes, the kids tune you out with the same reminder, so I switch to 5 more jumps if your child likes to jump into the pool, 3 more laps, or 5 more splashes. After that, leave the pool, take the towel, etc.

4. Always, always, always cheer them when they follow your instructions (entirely or partially). Remind them of their good behaviors at bedtime or when they calm so they will continue good behaviors or have the opportunity to express their needs in a less intense situation. Use this time to establish new rules that are set by both you and your child.

When setting consequences, sometimes you have to follow through what you said you were going to do ... you set firm boundaries and stick with them, your child knows that you are serious and mean business. You keep your promise, they will do the same.

All the best!

You and your son can make a portable visual schedule ahead of time. Read it a few times before you go to the pool. Then tell him verbally that you are sticking to the visual schedule. If he doesn't follow his visual schedule, then you have a choice to make. You can give a little grace or you can go strictly by the schedule. I've found in the long run, it's better to go by the schedule as much as possible, or there is no reason to have one.

Hi
Time to go eat always worked with my son but with that said I always tried to set and agree on a time frame ahead of time with him.None of this was flawless but over time we could pull it off without a meltdown.
Good luck thats a tough one.

When my kids are outside playing, I give them a "countdown" like "10 more mins" then 5 then 1 more min then we will be all done outside. I use the motive "if we come inside like a big boy and big girl, we earn more outside time." If either of them fight me, they lose outside time. You could do the same with the pool. 8/10 it works for us, hope that helps!

Thanks for all the great input guys!