Hi Nessa - your post made me smile, and I came up with a thought for you. I used to swim as a kid, and we had lap counters for the long distances. As the swimmer approached the wall, someone would dangle it in the water so they could see it.
Could you maybe make a little book of laminated sheets held together with some rings that you could put on the pool deck, or hang in the water if you have your own lane? Each page could count up by however many laps you can count up to before changing (you said you have adhd, so it may be 2, or just 1.) How many laps do you try to target?
Heck, even easier than laminating sheets, you could cut sheets of plastic and write on them in permanent marker. For fun you could also put smiley faces or short phrases on them to motivate you.
I know that this is long after the fact, but this is what I do. (I also forget easily.) I count lengths rather than laps, as it is helpful to remember that one end of the pool is the odd end and the other is the even end. I do my 60 lengths in three groups of 20. It is easy enough to remember where one is, when counting to 20. If, after 40, I think I might instead be at 60, I have only to look at the clock and realize that oops... I have 20 more to go.
I create a set to do in the pool such as 10 x 100 yds and so I only have to count to 10 and not worry. If I am particularly motivated I write out a whole workout before hand. I have ADHD too so I can't just get in and swim 1-2 miles without some sort of counting reference. Good luck with your timex though, I have never used one!