A few raindrops will settle out and be separated in your water filter. At worst case you will get a 'Water in fuel ' check light.
If you do, stop as soon as it is safe, , open the drain on the bottom of the filter and let it drain until the droplets 'sheet off' no longer 'beading off ' as droplets. A few pumps on the primer pump will help.
Use of a diesel fuel conditioner like Howes, Sea Foam or Diesel Service will dissipate water , for dissipation, not ridding the water.
Even treated dissipated water is harmful to common rail HPS diesel pumps.
Avoid the issue, deal with it, do not try to 'treat'' it away.