It sounds like your mast is water tight but the only way to really be certain is to put your mast entirely under water and look for air bubbles.
I removed my old mast base and taped the new base into place. I drilled holes at the 1, 5, 7 and 11 o'clock positions in the mast. Make sure your new holes are no higher than the old holes. Also, use a drill punch to indent into the mast prevent your drill bit from walking across the curved surface. Then drill the mast and the base at the same time.
I am not certain about the difference between Monel and SS rivets but I know that Monel is a proprietary SS, so technically, both Monel and SS rivets are SS. I don't think it makes a difference on the mast base since the only real stress on the rivets are when the mast is being raised. I put SS rivets in my mast and I have not had any problems for almost 2 years now.
2006 F18 Capricorn
Keep the pointy side up!