It's a serious question, so some interesting advise and information.
since you are ten years older this is less likely to be a problem like before!
often testicles tend to go into the body from where they were launched in foetal development under extreme cold, back into the cavity of the body, hence the joke "The water was so cold I got two lumps in my throat".
In adolescence and before this can also happen spontaneously and sometimes a testicle will not descend into its proper position.
As you pass into maturity hopefully things are larger than they were and the support muscles ligaments are also developed so this is less likely.
All Billy's suggestions are good but some normal cold shrinkage will still occur
our body does everything it can to keep a standard level of temperature so that sperm is produced normally. In hot weather there is the Chinese temperature control of One Hung Low, but both do!
A good fitting wetsuit is also answer. If it does happen again and it concerns you talk to your doctor.
Finally an interesting fact in an eastern martial art discipline one of the ways the warrior would frighten his enemy was to wear a uniform with no crutch in adoption the initial poses of a fight he would use muscular control to draw his testicles inside his body.
Scary but true
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun!