I would certainly say that adult disposable diapers would be noticeable under swimming shorts.
My 3 year old son who is not potty trained wears diapers all the time and wears disposable diapers under his swim shorts when he's on the beach or outside in the yard and yes they are noticeable under and through his swimming shorts.
But me and his mom knows he wears the diapers and can easily tell especially when he wets the diaper and it swells up.
But if I was not looking for the diaper in the pants and didn't know you were wearing adult diapers under your swimming shorts then I likely would not notice it.
So yes adult disposable diapers are likely noticeable in the swimming shorts but for most people they would not notice it.
If you need to wear adult diapers then do so and don't worry about what others think.
Many adults, older kids and teens wear diapers for incontinence and nobody really says anything even if they notice.