Good conservation stewardship is to use clear, clean, cool water.
Flirt with warm water at your peril.
Not worth soaking any stamp on coloured paper, observe patience and seek another copy. You may cut around the stamp, 5mm border and scan
if you wish.
Neutralising foxing can be done, Google philately foxing and get advice from reputed Philatelic conservators.
Whilst a stamp is in cool water, can be the best time to see a watermark. Otherwise use Watermark fluid.