Jamie,
If you changed to an internal system it would be advisable to seal the mast above the internal plate that the blocks hang off.
That is easy to do with a couple blocks of foam and some silicon.
Having said that it really it just a precaution as I have never been able to get the mast to go into the water much past the hounds (in a capsize) let alone turtle. It is quite surprising how bouyant they are.
Everyone sailing pleasure boats around with a bubble on the top of their mast could probably just change to this shaped mast section. A good safety measure as well as being very fast.
Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!