If the snorkle is designed properly and does not restrict airflow then it may enhance performance but in most cases the increase would be hard to notice. Engine longevity is more what it is about.
There are rubber boots that resemble a cows udder that can be used to help protect the dizzy from water ingress. Look on the brit parts houses websites for them.
One thing that you don't want to do is silicone your distributor cap to the base and gum up the works with a lot of sealant. This approach is a huge mistake.
If you are facing a particularly deep crossing, it may be prudent to remove the fan belt temporarily for the crossing. If you have electric fans in lieu of a vicous fan, then you can rig a switch to disable the fans for water crossings.
Proper wading technique i.e. creating a bow wave is the best defense against soaked electrics in the long run.
There is a good write up here on wading.