[DonC]

On average I probably drive my car a bit more than most. Have driven through my share of wet and can back up those that talk about "as long as you're moving at something above about 40 mph you should stay "pretty dry".
I've driven in rain and hail mix and hail hurts. Rain will also hurt in free fall but those are speeds that you usually won't see in a Cobra.
Water will find it's way onto your left knee from the door channel and your left shoulder will probably get a little damp from spray around the wind wings. If the rain lasts more than a about 15 minutes you'll probably get some blow-by on the inside of your windshield. I carry an 8" squeegee behind the passenger seat that I can get to for those times.
As with anything else "your mileage may differ" but overall I don't worry about it excessively. I always carry my soft top and side curtains in the trunk and if I see it coming and there's an opportunity to do so I'll put the top on. Takes about 5 minutes and does a good job.
Keep in mind these cars are based on a British design and by law they have to leak. Putting some drain holes in the floor isn't a bad idea. Put removable rubber plugs in them for normal driving and just pull them out and let them drain if the floor ever gets excessively wet.
X2 about the loss of traction on wet pavement and hydroplaning but, again, try not to get too wrapped around the axle about it. You KNOW it's a factor so adjust your driving accordingly. If you're driving on reasonably regular treaded tires you should be o.k., if you're street driving on something akin to drag slicks, not so much.
While I generally don't take the car out when I know it's going to rain I don't worry about it overly much if I get caught in it.
DonC