Maplin part number RE90X, it's a thick, waterproof, nonconductive silicon grease. First make sure it's all dry, then spread around and behind the seal on the end of the plug lead, and squidge it in. Make sure it's seated right in. Once you've done this, you can cross rivers

But, if your plug boot's damaged or not seated right it'll not help.
SV650Racer's right that clearing the drain hole, if it's blocked, can help. It's not a total fix though in my experience, no use in realyl heavy conditions- the plug doesn't have to be full of water, just sufficiently wet, the drain hole helps this but once it's wet it's wet and draining water away doesn't help much.