Under your swimbriefs

[Torchwatch]

Some of the Koala brand suits are designed to have a butt plug inserted.
If you've not used butt plugs before start small, use lots of lube and be gentle.
If it feels comfortable for you leave it in and enjoy it, you'll feel it every time you move.
Any anal sex toy should be flanged so they cannot be drawn up inside you and lost. Hospitals have collections of mobile phones, cigar cases etc that that have been surgically removed.

Cock rings and ball stretchers could be worn under swim briefs and swim thongs but they will leave a telling bulge. Make sure they are comfortable and do not restrict blood flow. Don't wear for too long in one go.