It makes it hardly run and I had to dry the cap!

silicone.

Just around the cap mating surface?

yup thats the only place water can get inside.. Also use dielectric grease on your plug wires and top of cap.

I also got this from another post and thought it was clever:

 

Take Latex/Nitrile glove.

Snip off just the end of each finger, push plug wires through and plug into distributer (4-plugs, 1-coil wire) then pull glove down over cap and zip tie close at base also zip tie each finger end.

Drive slower thru the puddles